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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-04 12:19:15 +0000 |
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| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-04 12:19:15 +0000 |
| commit | 6eb9c5a5657d1fe77b55cc261450f3538d35a94d (patch) | |
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Adding upstream version 13.4.upstream/13.4upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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diff --git a/doc/src/sgml/html/functions-matching.html b/doc/src/sgml/html/functions-matching.html new file mode 100644 index 0000000..2d0c524 --- /dev/null +++ b/doc/src/sgml/html/functions-matching.html @@ -0,0 +1,1248 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>9.7. Pattern Matching</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="functions-bitstring.html" title="9.6. Bit String Functions and Operators" /><link rel="next" href="functions-formatting.html" title="9.8. Data Type Formatting Functions" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">9.7. Pattern Matching</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="functions-bitstring.html" title="9.6. Bit String Functions and Operators">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="functions.html" title="Chapter 9. Functions and Operators">Up</a></td><th width="60%" align="center">Chapter 9. Functions and Operators</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="functions-formatting.html" title="9.8. Data Type Formatting Functions">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="FUNCTIONS-MATCHING"><div class="titlepage"><div><div><h2 class="title" style="clear: both">9.7. Pattern Matching</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="functions-matching.html#FUNCTIONS-LIKE">9.7.1. <code class="function">LIKE</code></a></span></dt><dt><span class="sect2"><a href="functions-matching.html#FUNCTIONS-SIMILARTO-REGEXP">9.7.2. <code class="function">SIMILAR TO</code> Regular Expressions</a></span></dt><dt><span class="sect2"><a href="functions-matching.html#FUNCTIONS-POSIX-REGEXP">9.7.3. <acronym class="acronym">POSIX</acronym> Regular Expressions</a></span></dt></dl></div><a id="id-1.5.8.13.2" class="indexterm"></a><p> + There are three separate approaches to pattern matching provided + by <span class="productname">PostgreSQL</span>: the traditional + <acronym class="acronym">SQL</acronym> <code class="function">LIKE</code> operator, the + more recent <code class="function">SIMILAR TO</code> operator (added in + SQL:1999), and <acronym class="acronym">POSIX</acronym>-style regular + expressions. Aside from the basic <span class="quote">“<span class="quote">does this string match + this pattern?</span>”</span> operators, functions are available to extract + or replace matching substrings and to split a string at matching + locations. + </p><div class="tip"><h3 class="title">Tip</h3><p> + If you have pattern matching needs that go beyond this, + consider writing a user-defined function in Perl or Tcl. + </p></div><div class="caution"><h3 class="title">Caution</h3><p> + While most regular-expression searches can be executed very quickly, + regular expressions can be contrived that take arbitrary amounts of + time and memory to process. Be wary of accepting regular-expression + search patterns from hostile sources. If you must do so, it is + advisable to impose a statement timeout. + </p><p> + Searches using <code class="function">SIMILAR TO</code> patterns have the same + security hazards, since <code class="function">SIMILAR TO</code> provides many + of the same capabilities as <acronym class="acronym">POSIX</acronym>-style regular + expressions. + </p><p> + <code class="function">LIKE</code> searches, being much simpler than the other + two options, are safer to use with possibly-hostile pattern sources. + </p></div><p> + The pattern matching operators of all three kinds do not support + nondeterministic collations. If required, apply a different collation to + the expression to work around this limitation. + </p><div class="sect2" id="FUNCTIONS-LIKE"><div class="titlepage"><div><div><h3 class="title">9.7.1. <code class="function">LIKE</code></h3></div></div></div><a id="id-1.5.8.13.7.2" class="indexterm"></a><pre class="synopsis"> +<em class="replaceable"><code>string</code></em> LIKE <em class="replaceable"><code>pattern</code></em> [<span class="optional">ESCAPE <em class="replaceable"><code>escape-character</code></em></span>] +<em class="replaceable"><code>string</code></em> NOT LIKE <em class="replaceable"><code>pattern</code></em> [<span class="optional">ESCAPE <em class="replaceable"><code>escape-character</code></em></span>] +</pre><p> + The <code class="function">LIKE</code> expression returns true if the + <em class="replaceable"><code>string</code></em> matches the supplied + <em class="replaceable"><code>pattern</code></em>. (As + expected, the <code class="function">NOT LIKE</code> expression returns + false if <code class="function">LIKE</code> returns true, and vice versa. + An equivalent expression is + <code class="literal">NOT (<em class="replaceable"><code>string</code></em> LIKE + <em class="replaceable"><code>pattern</code></em>)</code>.) + </p><p> + If <em class="replaceable"><code>pattern</code></em> does not contain percent + signs or underscores, then the pattern only represents the string + itself; in that case <code class="function">LIKE</code> acts like the + equals operator. An underscore (<code class="literal">_</code>) in + <em class="replaceable"><code>pattern</code></em> stands for (matches) any single + character; a percent sign (<code class="literal">%</code>) matches any sequence + of zero or more characters. + </p><p> + Some examples: +</p><pre class="programlisting"> +'abc' LIKE 'abc' <em class="lineannotation"><span class="lineannotation">true</span></em> +'abc' LIKE 'a%' <em class="lineannotation"><span class="lineannotation">true</span></em> +'abc' LIKE '_b_' <em class="lineannotation"><span class="lineannotation">true</span></em> +'abc' LIKE 'c' <em class="lineannotation"><span class="lineannotation">false</span></em> +</pre><p> + </p><p> + <code class="function">LIKE</code> pattern matching always covers the entire + string. Therefore, if it's desired to match a sequence anywhere within + a string, the pattern must start and end with a percent sign. + </p><p> + To match a literal underscore or percent sign without matching + other characters, the respective character in + <em class="replaceable"><code>pattern</code></em> must be + preceded by the escape character. The default escape + character is the backslash but a different one can be selected by + using the <code class="literal">ESCAPE</code> clause. To match the escape + character itself, write two escape characters. + </p><div class="note"><h3 class="title">Note</h3><p> + If you have <a class="xref" href="runtime-config-compatible.html#GUC-STANDARD-CONFORMING-STRINGS">standard_conforming_strings</a> turned off, + any backslashes you write in literal string constants will need to be + doubled. See <a class="xref" href="sql-syntax-lexical.html#SQL-SYNTAX-STRINGS" title="4.1.2.1. String Constants">Section 4.1.2.1</a> for more information. + </p></div><p> + It's also possible to select no escape character by writing + <code class="literal">ESCAPE ''</code>. This effectively disables the + escape mechanism, which makes it impossible to turn off the + special meaning of underscore and percent signs in the pattern. + </p><p> + According to the SQL standard, omitting <code class="literal">ESCAPE</code> + means there is no escape character (rather than defaulting to a + backslash), and a zero-length <code class="literal">ESCAPE</code> value is + disallowed. <span class="productname">PostgreSQL</span>'s behavior in + this regard is therefore slightly nonstandard. + </p><p> + The key word <code class="token">ILIKE</code> can be used instead of + <code class="token">LIKE</code> to make the match case-insensitive according + to the active locale. This is not in the <acronym class="acronym">SQL</acronym> standard but is a + <span class="productname">PostgreSQL</span> extension. + </p><p> + The operator <code class="literal">~~</code> is equivalent to + <code class="function">LIKE</code>, and <code class="literal">~~*</code> corresponds to + <code class="function">ILIKE</code>. There are also + <code class="literal">!~~</code> and <code class="literal">!~~*</code> operators that + represent <code class="function">NOT LIKE</code> and <code class="function">NOT + ILIKE</code>, respectively. All of these operators are + <span class="productname">PostgreSQL</span>-specific. You may see these + operator names in <code class="command">EXPLAIN</code> output and similar + places, since the parser actually translates <code class="function">LIKE</code> + et al. to these operators. + </p><p> + The phrases <code class="function">LIKE</code>, <code class="function">ILIKE</code>, + <code class="function">NOT LIKE</code>, and <code class="function">NOT ILIKE</code> are + generally treated as operators + in <span class="productname">PostgreSQL</span> syntax; for example they can + be used in <em class="replaceable"><code>expression</code></em> + <em class="replaceable"><code>operator</code></em> ANY + (<em class="replaceable"><code>subquery</code></em>) constructs, although + an <code class="literal">ESCAPE</code> clause cannot be included there. In some + obscure cases it may be necessary to use the underlying operator names + instead. + </p><p> + Also see the prefix operator <code class="literal">^@</code> and corresponding + <code class="function">starts_with</code> function, which are useful in cases + where simply matching the beginning of a string is needed. + </p></div><div class="sect2" id="FUNCTIONS-SIMILARTO-REGEXP"><div class="titlepage"><div><div><h3 class="title">9.7.2. <code class="function">SIMILAR TO</code> Regular Expressions</h3></div></div></div><a id="id-1.5.8.13.8.2" class="indexterm"></a><a id="id-1.5.8.13.8.3" class="indexterm"></a><a id="id-1.5.8.13.8.4" class="indexterm"></a><pre class="synopsis"> +<em class="replaceable"><code>string</code></em> SIMILAR TO <em class="replaceable"><code>pattern</code></em> [<span class="optional">ESCAPE <em class="replaceable"><code>escape-character</code></em></span>] +<em class="replaceable"><code>string</code></em> NOT SIMILAR TO <em class="replaceable"><code>pattern</code></em> [<span class="optional">ESCAPE <em class="replaceable"><code>escape-character</code></em></span>] +</pre><p> + The <code class="function">SIMILAR TO</code> operator returns true or + false depending on whether its pattern matches the given string. + It is similar to <code class="function">LIKE</code>, except that it + interprets the pattern using the SQL standard's definition of a + regular expression. SQL regular expressions are a curious cross + between <code class="function">LIKE</code> notation and common (POSIX) regular + expression notation. + </p><p> + Like <code class="function">LIKE</code>, the <code class="function">SIMILAR TO</code> + operator succeeds only if its pattern matches the entire string; + this is unlike common regular expression behavior where the pattern + can match any part of the string. + Also like + <code class="function">LIKE</code>, <code class="function">SIMILAR TO</code> uses + <code class="literal">_</code> and <code class="literal">%</code> as wildcard characters denoting + any single character and any string, respectively (these are + comparable to <code class="literal">.</code> and <code class="literal">.*</code> in POSIX regular + expressions). + </p><p> + In addition to these facilities borrowed from <code class="function">LIKE</code>, + <code class="function">SIMILAR TO</code> supports these pattern-matching + metacharacters borrowed from POSIX regular expressions: + + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + <code class="literal">|</code> denotes alternation (either of two alternatives). + </p></li><li class="listitem"><p> + <code class="literal">*</code> denotes repetition of the previous item zero + or more times. + </p></li><li class="listitem"><p> + <code class="literal">+</code> denotes repetition of the previous item one + or more times. + </p></li><li class="listitem"><p> + <code class="literal">?</code> denotes repetition of the previous item zero + or one time. + </p></li><li class="listitem"><p> + <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">}</code> denotes repetition + of the previous item exactly <em class="replaceable"><code>m</code></em> times. + </p></li><li class="listitem"><p> + <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">,}</code> denotes repetition + of the previous item <em class="replaceable"><code>m</code></em> or more times. + </p></li><li class="listitem"><p> + <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">,</code><em class="replaceable"><code>n</code></em><code class="literal">}</code> + denotes repetition of the previous item at least <em class="replaceable"><code>m</code></em> and + not more than <em class="replaceable"><code>n</code></em> times. + </p></li><li class="listitem"><p> + Parentheses <code class="literal">()</code> can be used to group items into + a single logical item. + </p></li><li class="listitem"><p> + A bracket expression <code class="literal">[...]</code> specifies a character + class, just as in POSIX regular expressions. + </p></li></ul></div><p> + + Notice that the period (<code class="literal">.</code>) is not a metacharacter + for <code class="function">SIMILAR TO</code>. + </p><p> + As with <code class="function">LIKE</code>, a backslash disables the special + meaning of any of these metacharacters. A different escape character + can be specified with <code class="literal">ESCAPE</code>, or the escape + capability can be disabled by writing <code class="literal">ESCAPE ''</code>. + </p><p> + According to the SQL standard, omitting <code class="literal">ESCAPE</code> + means there is no escape character (rather than defaulting to a + backslash), and a zero-length <code class="literal">ESCAPE</code> value is + disallowed. <span class="productname">PostgreSQL</span>'s behavior in + this regard is therefore slightly nonstandard. + </p><p> + Another nonstandard extension is that following the escape character + with a letter or digit provides access to the escape sequences + defined for POSIX regular expressions; see + <a class="xref" href="functions-matching.html#POSIX-CHARACTER-ENTRY-ESCAPES-TABLE" title="Table 9.20. Regular Expression Character-Entry Escapes">Table 9.20</a>, + <a class="xref" href="functions-matching.html#POSIX-CLASS-SHORTHAND-ESCAPES-TABLE" title="Table 9.21. Regular Expression Class-Shorthand Escapes">Table 9.21</a>, and + <a class="xref" href="functions-matching.html#POSIX-CONSTRAINT-ESCAPES-TABLE" title="Table 9.22. Regular Expression Constraint Escapes">Table 9.22</a> below. + </p><p> + Some examples: +</p><pre class="programlisting"> +'abc' SIMILAR TO 'abc' <em class="lineannotation"><span class="lineannotation">true</span></em> +'abc' SIMILAR TO 'a' <em class="lineannotation"><span class="lineannotation">false</span></em> +'abc' SIMILAR TO '%(b|d)%' <em class="lineannotation"><span class="lineannotation">true</span></em> +'abc' SIMILAR TO '(b|c)%' <em class="lineannotation"><span class="lineannotation">false</span></em> +'-abc-' SIMILAR TO '%\mabc\M%' <em class="lineannotation"><span class="lineannotation">true</span></em> +'xabcy' SIMILAR TO '%\mabc\M%' <em class="lineannotation"><span class="lineannotation">false</span></em> +</pre><p> + </p><p> + The <code class="function">substring</code> function with three parameters + provides extraction of a substring that matches an SQL + regular expression pattern. The function can be written according + to SQL99 syntax: +</p><pre class="synopsis"> +substring(<em class="replaceable"><code>string</code></em> from <em class="replaceable"><code>pattern</code></em> for <em class="replaceable"><code>escape-character</code></em>) +</pre><p> + or as a plain three-argument function: +</p><pre class="synopsis"> +substring(<em class="replaceable"><code>string</code></em>, <em class="replaceable"><code>pattern</code></em>, <em class="replaceable"><code>escape-character</code></em>) +</pre><p> + As with <code class="literal">SIMILAR TO</code>, the + specified pattern must match the entire data string, or else the + function fails and returns null. To indicate the part of the + pattern for which the matching data sub-string is of interest, + the pattern should contain + two occurrences of the escape character followed by a double quote + (<code class="literal">"</code>). + The text matching the portion of the pattern + between these separators is returned when the match is successful. + </p><p> + The escape-double-quote separators actually + divide <code class="function">substring</code>'s pattern into three independent + regular expressions; for example, a vertical bar (<code class="literal">|</code>) + in any of the three sections affects only that section. Also, the first + and third of these regular expressions are defined to match the smallest + possible amount of text, not the largest, when there is any ambiguity + about how much of the data string matches which pattern. (In POSIX + parlance, the first and third regular expressions are forced to be + non-greedy.) + </p><p> + As an extension to the SQL standard, <span class="productname">PostgreSQL</span> + allows there to be just one escape-double-quote separator, in which case + the third regular expression is taken as empty; or no separators, in which + case the first and third regular expressions are taken as empty. + </p><p> + Some examples, with <code class="literal">#"</code> delimiting the return string: +</p><pre class="programlisting"> +substring('foobar' from '%#"o_b#"%' for '#') <em class="lineannotation"><span class="lineannotation">oob</span></em> +substring('foobar' from '#"o_b#"%' for '#') <em class="lineannotation"><span class="lineannotation">NULL</span></em> +</pre><p> + </p></div><div class="sect2" id="FUNCTIONS-POSIX-REGEXP"><div class="titlepage"><div><div><h3 class="title">9.7.3. <acronym class="acronym">POSIX</acronym> Regular Expressions</h3></div></div></div><a id="id-1.5.8.13.9.2" class="indexterm"></a><a id="id-1.5.8.13.9.3" class="indexterm"></a><a id="id-1.5.8.13.9.4" class="indexterm"></a><a id="id-1.5.8.13.9.5" class="indexterm"></a><a id="id-1.5.8.13.9.6" class="indexterm"></a><a id="id-1.5.8.13.9.7" class="indexterm"></a><a id="id-1.5.8.13.9.8" class="indexterm"></a><p> + <a class="xref" href="functions-matching.html#FUNCTIONS-POSIX-TABLE" title="Table 9.16. Regular Expression Match Operators">Table 9.16</a> lists the available + operators for pattern matching using POSIX regular expressions. + </p><div class="table" id="FUNCTIONS-POSIX-TABLE"><p class="title"><strong>Table 9.16. Regular Expression Match Operators</strong></p><div class="table-contents"><table class="table" summary="Regular Expression Match Operators" border="1"><colgroup><col /></colgroup><thead><tr><th class="func_table_entry"><p class="func_signature"> + Operator + </p> + <p> + Description + </p> + <p> + Example(s) + </p></th></tr></thead><tbody><tr><td class="func_table_entry"><p class="func_signature"> + <code class="type">text</code> <code class="literal">~</code> <code class="type">text</code> + → <code class="returnvalue">boolean</code> + </p> + <p> + String matches regular expression, case sensitively + </p> + <p> + <code class="literal">'thomas' ~ 't.*ma'</code> + → <code class="returnvalue">t</code> + </p></td></tr><tr><td class="func_table_entry"><p class="func_signature"> + <code class="type">text</code> <code class="literal">~*</code> <code class="type">text</code> + → <code class="returnvalue">boolean</code> + </p> + <p> + String matches regular expression, case insensitively + </p> + <p> + <code class="literal">'thomas' ~* 'T.*ma'</code> + → <code class="returnvalue">t</code> + </p></td></tr><tr><td class="func_table_entry"><p class="func_signature"> + <code class="type">text</code> <code class="literal">!~</code> <code class="type">text</code> + → <code class="returnvalue">boolean</code> + </p> + <p> + String does not match regular expression, case sensitively + </p> + <p> + <code class="literal">'thomas' !~ 't.*max'</code> + → <code class="returnvalue">t</code> + </p></td></tr><tr><td class="func_table_entry"><p class="func_signature"> + <code class="type">text</code> <code class="literal">!~*</code> <code class="type">text</code> + → <code class="returnvalue">boolean</code> + </p> + <p> + String does not match regular expression, case insensitively + </p> + <p> + <code class="literal">'thomas' !~* 'T.*ma'</code> + → <code class="returnvalue">f</code> + </p></td></tr></tbody></table></div></div><br class="table-break" /><p> + <acronym class="acronym">POSIX</acronym> regular expressions provide a more + powerful means for pattern matching than the <code class="function">LIKE</code> and + <code class="function">SIMILAR TO</code> operators. + Many Unix tools such as <code class="command">egrep</code>, + <code class="command">sed</code>, or <code class="command">awk</code> use a pattern + matching language that is similar to the one described here. + </p><p> + A regular expression is a character sequence that is an + abbreviated definition of a set of strings (a <em class="firstterm">regular + set</em>). A string is said to match a regular expression + if it is a member of the regular set described by the regular + expression. As with <code class="function">LIKE</code>, pattern characters + match string characters exactly unless they are special characters + in the regular expression language — but regular expressions use + different special characters than <code class="function">LIKE</code> does. + Unlike <code class="function">LIKE</code> patterns, a + regular expression is allowed to match anywhere within a string, unless + the regular expression is explicitly anchored to the beginning or + end of the string. + </p><p> + Some examples: +</p><pre class="programlisting"> +'abcd' ~ 'bc' <em class="lineannotation"><span class="lineannotation">true</span></em> +'abcd' ~ 'a.c' <em class="lineannotation"><span class="lineannotation">true — dot matches any character</span></em> +'abcd' ~ 'a.*d' <em class="lineannotation"><span class="lineannotation">true — <code class="literal">*</code> repeats the preceding pattern item</span></em> +'abcd' ~ '(b|x)' <em class="lineannotation"><span class="lineannotation">true — <code class="literal">|</code> means OR, parentheses group</span></em> +'abcd' ~ '^a' <em class="lineannotation"><span class="lineannotation">true — <code class="literal">^</code> anchors to start of string</span></em> +'abcd' ~ '^(b|c)' <em class="lineannotation"><span class="lineannotation">false — would match except for anchoring</span></em> +</pre><p> + </p><p> + The <acronym class="acronym">POSIX</acronym> pattern language is described in much + greater detail below. + </p><p> + The <code class="function">substring</code> function with two parameters, + <code class="function">substring(<em class="replaceable"><code>string</code></em> from + <em class="replaceable"><code>pattern</code></em>)</code>, provides extraction of a + substring + that matches a POSIX regular expression pattern. It returns null if + there is no match, otherwise the first portion of the text that matched the + pattern. But if the pattern contains any parentheses, the portion + of the text that matched the first parenthesized subexpression (the + one whose left parenthesis comes first) is + returned. You can put parentheses around the whole expression + if you want to use parentheses within it without triggering this + exception. If you need parentheses in the pattern before the + subexpression you want to extract, see the non-capturing parentheses + described below. + </p><p> + Some examples: +</p><pre class="programlisting"> +substring('foobar' from 'o.b') <em class="lineannotation"><span class="lineannotation">oob</span></em> +substring('foobar' from 'o(.)b') <em class="lineannotation"><span class="lineannotation">o</span></em> +</pre><p> + </p><p> + The <code class="function">regexp_replace</code> function provides substitution of + new text for substrings that match POSIX regular expression patterns. + It has the syntax + <code class="function">regexp_replace</code>(<em class="replaceable"><code>source</code></em>, + <em class="replaceable"><code>pattern</code></em>, <em class="replaceable"><code>replacement</code></em> + [<span class="optional">, <em class="replaceable"><code>flags</code></em> </span>]). + The <em class="replaceable"><code>source</code></em> string is returned unchanged if + there is no match to the <em class="replaceable"><code>pattern</code></em>. If there is a + match, the <em class="replaceable"><code>source</code></em> string is returned with the + <em class="replaceable"><code>replacement</code></em> string substituted for the matching + substring. The <em class="replaceable"><code>replacement</code></em> string can contain + <code class="literal">\</code><em class="replaceable"><code>n</code></em>, where <em class="replaceable"><code>n</code></em> is 1 + through 9, to indicate that the source substring matching the + <em class="replaceable"><code>n</code></em>'th parenthesized subexpression of the pattern should be + inserted, and it can contain <code class="literal">\&</code> to indicate that the + substring matching the entire pattern should be inserted. Write + <code class="literal">\\</code> if you need to put a literal backslash in the replacement + text. + The <em class="replaceable"><code>flags</code></em> parameter is an optional text + string containing zero or more single-letter flags that change the + function's behavior. Flag <code class="literal">i</code> specifies case-insensitive + matching, while flag <code class="literal">g</code> specifies replacement of each matching + substring rather than only the first one. Supported flags (though + not <code class="literal">g</code>) are + described in <a class="xref" href="functions-matching.html#POSIX-EMBEDDED-OPTIONS-TABLE" title="Table 9.24. ARE Embedded-Option Letters">Table 9.24</a>. + </p><p> + Some examples: +</p><pre class="programlisting"> +regexp_replace('foobarbaz', 'b..', 'X') + <em class="lineannotation"><span class="lineannotation">fooXbaz</span></em> +regexp_replace('foobarbaz', 'b..', 'X', 'g') + <em class="lineannotation"><span class="lineannotation">fooXX</span></em> +regexp_replace('foobarbaz', 'b(..)', 'X\1Y', 'g') + <em class="lineannotation"><span class="lineannotation">fooXarYXazY</span></em> +</pre><p> + </p><p> + The <code class="function">regexp_match</code> function returns a text array of + captured substring(s) resulting from the first match of a POSIX + regular expression pattern to a string. It has the syntax + <code class="function">regexp_match</code>(<em class="replaceable"><code>string</code></em>, + <em class="replaceable"><code>pattern</code></em> [<span class="optional">, <em class="replaceable"><code>flags</code></em> </span>]). + If there is no match, the result is <code class="literal">NULL</code>. + If a match is found, and the <em class="replaceable"><code>pattern</code></em> contains no + parenthesized subexpressions, then the result is a single-element text + array containing the substring matching the whole pattern. + If a match is found, and the <em class="replaceable"><code>pattern</code></em> contains + parenthesized subexpressions, then the result is a text array + whose <em class="replaceable"><code>n</code></em>'th element is the substring matching + the <em class="replaceable"><code>n</code></em>'th parenthesized subexpression of + the <em class="replaceable"><code>pattern</code></em> (not counting <span class="quote">“<span class="quote">non-capturing</span>”</span> + parentheses; see below for details). + The <em class="replaceable"><code>flags</code></em> parameter is an optional text string + containing zero or more single-letter flags that change the function's + behavior. Supported flags are described + in <a class="xref" href="functions-matching.html#POSIX-EMBEDDED-OPTIONS-TABLE" title="Table 9.24. ARE Embedded-Option Letters">Table 9.24</a>. + </p><p> + Some examples: +</p><pre class="programlisting"> +SELECT regexp_match('foobarbequebaz', 'bar.*que'); + regexp_match +-------------- + {barbeque} +(1 row) + +SELECT regexp_match('foobarbequebaz', '(bar)(beque)'); + regexp_match +-------------- + {bar,beque} +(1 row) +</pre><p> + In the common case where you just want the whole matching substring + or <code class="literal">NULL</code> for no match, write something like +</p><pre class="programlisting"> +SELECT (regexp_match('foobarbequebaz', 'bar.*que'))[1]; + regexp_match +-------------- + barbeque +(1 row) +</pre><p> + </p><p> + The <code class="function">regexp_matches</code> function returns a set of text arrays + of captured substring(s) resulting from matching a POSIX regular + expression pattern to a string. It has the same syntax as + <code class="function">regexp_match</code>. + This function returns no rows if there is no match, one row if there is + a match and the <code class="literal">g</code> flag is not given, or <em class="replaceable"><code>N</code></em> + rows if there are <em class="replaceable"><code>N</code></em> matches and the <code class="literal">g</code> flag + is given. Each returned row is a text array containing the whole + matched substring or the substrings matching parenthesized + subexpressions of the <em class="replaceable"><code>pattern</code></em>, just as described above + for <code class="function">regexp_match</code>. + <code class="function">regexp_matches</code> accepts all the flags shown + in <a class="xref" href="functions-matching.html#POSIX-EMBEDDED-OPTIONS-TABLE" title="Table 9.24. ARE Embedded-Option Letters">Table 9.24</a>, plus + the <code class="literal">g</code> flag which commands it to return all matches, not + just the first one. + </p><p> + Some examples: +</p><pre class="programlisting"> +SELECT regexp_matches('foo', 'not there'); + regexp_matches +---------------- +(0 rows) + +SELECT regexp_matches('foobarbequebazilbarfbonk', '(b[^b]+)(b[^b]+)', 'g'); + regexp_matches +---------------- + {bar,beque} + {bazil,barf} +(2 rows) +</pre><p> + </p><div class="tip"><h3 class="title">Tip</h3><p> + In most cases <code class="function">regexp_matches()</code> should be used with + the <code class="literal">g</code> flag, since if you only want the first match, it's + easier and more efficient to use <code class="function">regexp_match()</code>. + However, <code class="function">regexp_match()</code> only exists + in <span class="productname">PostgreSQL</span> version 10 and up. When working in older + versions, a common trick is to place a <code class="function">regexp_matches()</code> + call in a sub-select, for example: +</p><pre class="programlisting"> +SELECT col1, (SELECT regexp_matches(col2, '(bar)(beque)')) FROM tab; +</pre><p> + This produces a text array if there's a match, or <code class="literal">NULL</code> if + not, the same as <code class="function">regexp_match()</code> would do. Without the + sub-select, this query would produce no output at all for table rows + without a match, which is typically not the desired behavior. + </p></div><p> + The <code class="function">regexp_split_to_table</code> function splits a string using a POSIX + regular expression pattern as a delimiter. It has the syntax + <code class="function">regexp_split_to_table</code>(<em class="replaceable"><code>string</code></em>, <em class="replaceable"><code>pattern</code></em> + [<span class="optional">, <em class="replaceable"><code>flags</code></em> </span>]). + If there is no match to the <em class="replaceable"><code>pattern</code></em>, the function returns the + <em class="replaceable"><code>string</code></em>. If there is at least one match, for each match it returns + the text from the end of the last match (or the beginning of the string) + to the beginning of the match. When there are no more matches, it + returns the text from the end of the last match to the end of the string. + The <em class="replaceable"><code>flags</code></em> parameter is an optional text string containing + zero or more single-letter flags that change the function's behavior. + <code class="function">regexp_split_to_table</code> supports the flags described in + <a class="xref" href="functions-matching.html#POSIX-EMBEDDED-OPTIONS-TABLE" title="Table 9.24. ARE Embedded-Option Letters">Table 9.24</a>. + </p><p> + The <code class="function">regexp_split_to_array</code> function behaves the same as + <code class="function">regexp_split_to_table</code>, except that <code class="function">regexp_split_to_array</code> + returns its result as an array of <code class="type">text</code>. It has the syntax + <code class="function">regexp_split_to_array</code>(<em class="replaceable"><code>string</code></em>, <em class="replaceable"><code>pattern</code></em> + [<span class="optional">, <em class="replaceable"><code>flags</code></em> </span>]). + The parameters are the same as for <code class="function">regexp_split_to_table</code>. + </p><p> + Some examples: +</p><pre class="programlisting"> + +SELECT foo FROM regexp_split_to_table('the quick brown fox jumps over the lazy dog', '\s+') AS foo; + foo +------- + the + quick + brown + fox + jumps + over + the + lazy + dog +(9 rows) + +SELECT regexp_split_to_array('the quick brown fox jumps over the lazy dog', '\s+'); + regexp_split_to_array +----------------------------------------------- + {the,quick,brown,fox,jumps,over,the,lazy,dog} +(1 row) + +SELECT foo FROM regexp_split_to_table('the quick brown fox', '\s*') AS foo; + foo +----- + t + h + e + q + u + i + c + k + b + r + o + w + n + f + o + x +(16 rows) +</pre><p> + </p><p> + As the last example demonstrates, the regexp split functions ignore + zero-length matches that occur at the start or end of the string + or immediately after a previous match. This is contrary to the strict + definition of regexp matching that is implemented by + <code class="function">regexp_match</code> and + <code class="function">regexp_matches</code>, but is usually the most convenient behavior + in practice. Other software systems such as Perl use similar definitions. + </p><div class="sect3" id="POSIX-SYNTAX-DETAILS"><div class="titlepage"><div><div><h4 class="title">9.7.3.1. Regular Expression Details</h4></div></div></div><p> + <span class="productname">PostgreSQL</span>'s regular expressions are implemented + using a software package written by Henry Spencer. Much of + the description of regular expressions below is copied verbatim from his + manual. + </p><p> + Regular expressions (<acronym class="acronym">RE</acronym>s), as defined in + <acronym class="acronym">POSIX</acronym> 1003.2, come in two forms: + <em class="firstterm">extended</em> <acronym class="acronym">RE</acronym>s or <acronym class="acronym">ERE</acronym>s + (roughly those of <code class="command">egrep</code>), and + <em class="firstterm">basic</em> <acronym class="acronym">RE</acronym>s or <acronym class="acronym">BRE</acronym>s + (roughly those of <code class="command">ed</code>). + <span class="productname">PostgreSQL</span> supports both forms, and + also implements some extensions + that are not in the POSIX standard, but have become widely used + due to their availability in programming languages such as Perl and Tcl. + <acronym class="acronym">RE</acronym>s using these non-POSIX extensions are called + <em class="firstterm">advanced</em> <acronym class="acronym">RE</acronym>s or <acronym class="acronym">ARE</acronym>s + in this documentation. AREs are almost an exact superset of EREs, + but BREs have several notational incompatibilities (as well as being + much more limited). + We first describe the ARE and ERE forms, noting features that apply + only to AREs, and then describe how BREs differ. + </p><div class="note"><h3 class="title">Note</h3><p> + <span class="productname">PostgreSQL</span> always initially presumes that a regular + expression follows the ARE rules. However, the more limited ERE or + BRE rules can be chosen by prepending an <em class="firstterm">embedded option</em> + to the RE pattern, as described in <a class="xref" href="functions-matching.html#POSIX-METASYNTAX" title="9.7.3.4. Regular Expression Metasyntax">Section 9.7.3.4</a>. + This can be useful for compatibility with applications that expect + exactly the <acronym class="acronym">POSIX</acronym> 1003.2 rules. + </p></div><p> + A regular expression is defined as one or more + <em class="firstterm">branches</em>, separated by + <code class="literal">|</code>. It matches anything that matches one of the + branches. + </p><p> + A branch is zero or more <em class="firstterm">quantified atoms</em> or + <em class="firstterm">constraints</em>, concatenated. + It matches a match for the first, followed by a match for the second, etc; + an empty branch matches the empty string. + </p><p> + A quantified atom is an <em class="firstterm">atom</em> possibly followed + by a single <em class="firstterm">quantifier</em>. + Without a quantifier, it matches a match for the atom. + With a quantifier, it can match some number of matches of the atom. + An <em class="firstterm">atom</em> can be any of the possibilities + shown in <a class="xref" href="functions-matching.html#POSIX-ATOMS-TABLE" title="Table 9.17. Regular Expression Atoms">Table 9.17</a>. + The possible quantifiers and their meanings are shown in + <a class="xref" href="functions-matching.html#POSIX-QUANTIFIERS-TABLE" title="Table 9.18. Regular Expression Quantifiers">Table 9.18</a>. + </p><p> + A <em class="firstterm">constraint</em> matches an empty string, but matches only when + specific conditions are met. A constraint can be used where an atom + could be used, except it cannot be followed by a quantifier. + The simple constraints are shown in + <a class="xref" href="functions-matching.html#POSIX-CONSTRAINTS-TABLE" title="Table 9.19. Regular Expression Constraints">Table 9.19</a>; + some more constraints are described later. + </p><div class="table" id="POSIX-ATOMS-TABLE"><p class="title"><strong>Table 9.17. Regular Expression Atoms</strong></p><div class="table-contents"><table class="table" summary="Regular Expression Atoms" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>Atom</th><th>Description</th></tr></thead><tbody><tr><td> <code class="literal">(</code><em class="replaceable"><code>re</code></em><code class="literal">)</code> </td><td> (where <em class="replaceable"><code>re</code></em> is any regular expression) + matches a match for + <em class="replaceable"><code>re</code></em>, with the match noted for possible reporting </td></tr><tr><td> <code class="literal">(?:</code><em class="replaceable"><code>re</code></em><code class="literal">)</code> </td><td> as above, but the match is not noted for reporting + (a <span class="quote">“<span class="quote">non-capturing</span>”</span> set of parentheses) + (AREs only) </td></tr><tr><td> <code class="literal">.</code> </td><td> matches any single character </td></tr><tr><td> <code class="literal">[</code><em class="replaceable"><code>chars</code></em><code class="literal">]</code> </td><td> a <em class="firstterm">bracket expression</em>, + matching any one of the <em class="replaceable"><code>chars</code></em> (see + <a class="xref" href="functions-matching.html#POSIX-BRACKET-EXPRESSIONS" title="9.7.3.2. Bracket Expressions">Section 9.7.3.2</a> for more detail) </td></tr><tr><td> <code class="literal">\</code><em class="replaceable"><code>k</code></em> </td><td> (where <em class="replaceable"><code>k</code></em> is a non-alphanumeric character) + matches that character taken as an ordinary character, + e.g., <code class="literal">\\</code> matches a backslash character </td></tr><tr><td> <code class="literal">\</code><em class="replaceable"><code>c</code></em> </td><td> where <em class="replaceable"><code>c</code></em> is alphanumeric + (possibly followed by other characters) + is an <em class="firstterm">escape</em>, see <a class="xref" href="functions-matching.html#POSIX-ESCAPE-SEQUENCES" title="9.7.3.3. Regular Expression Escapes">Section 9.7.3.3</a> + (AREs only; in EREs and BREs, this matches <em class="replaceable"><code>c</code></em>) </td></tr><tr><td> <code class="literal">{</code> </td><td> when followed by a character other than a digit, + matches the left-brace character <code class="literal">{</code>; + when followed by a digit, it is the beginning of a + <em class="replaceable"><code>bound</code></em> (see below) </td></tr><tr><td> <em class="replaceable"><code>x</code></em> </td><td> where <em class="replaceable"><code>x</code></em> is a single character with no other + significance, matches that character </td></tr></tbody></table></div></div><br class="table-break" /><p> + An RE cannot end with a backslash (<code class="literal">\</code>). + </p><div class="note"><h3 class="title">Note</h3><p> + If you have <a class="xref" href="runtime-config-compatible.html#GUC-STANDARD-CONFORMING-STRINGS">standard_conforming_strings</a> turned off, + any backslashes you write in literal string constants will need to be + doubled. See <a class="xref" href="sql-syntax-lexical.html#SQL-SYNTAX-STRINGS" title="4.1.2.1. String Constants">Section 4.1.2.1</a> for more information. + </p></div><div class="table" id="POSIX-QUANTIFIERS-TABLE"><p class="title"><strong>Table 9.18. Regular Expression Quantifiers</strong></p><div class="table-contents"><table class="table" summary="Regular Expression Quantifiers" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>Quantifier</th><th>Matches</th></tr></thead><tbody><tr><td> <code class="literal">*</code> </td><td> a sequence of 0 or more matches of the atom </td></tr><tr><td> <code class="literal">+</code> </td><td> a sequence of 1 or more matches of the atom </td></tr><tr><td> <code class="literal">?</code> </td><td> a sequence of 0 or 1 matches of the atom </td></tr><tr><td> <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">}</code> </td><td> a sequence of exactly <em class="replaceable"><code>m</code></em> matches of the atom </td></tr><tr><td> <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">,}</code> </td><td> a sequence of <em class="replaceable"><code>m</code></em> or more matches of the atom </td></tr><tr><td> + <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">,</code><em class="replaceable"><code>n</code></em><code class="literal">}</code> </td><td> a sequence of <em class="replaceable"><code>m</code></em> through <em class="replaceable"><code>n</code></em> + (inclusive) matches of the atom; <em class="replaceable"><code>m</code></em> cannot exceed + <em class="replaceable"><code>n</code></em> </td></tr><tr><td> <code class="literal">*?</code> </td><td> non-greedy version of <code class="literal">*</code> </td></tr><tr><td> <code class="literal">+?</code> </td><td> non-greedy version of <code class="literal">+</code> </td></tr><tr><td> <code class="literal">??</code> </td><td> non-greedy version of <code class="literal">?</code> </td></tr><tr><td> <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">}?</code> </td><td> non-greedy version of <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">}</code> </td></tr><tr><td> <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">,}?</code> </td><td> non-greedy version of <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">,}</code> </td></tr><tr><td> + <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">,</code><em class="replaceable"><code>n</code></em><code class="literal">}?</code> </td><td> non-greedy version of <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">,</code><em class="replaceable"><code>n</code></em><code class="literal">}</code> </td></tr></tbody></table></div></div><br class="table-break" /><p> + The forms using <code class="literal">{</code><em class="replaceable"><code>...</code></em><code class="literal">}</code> + are known as <em class="firstterm">bounds</em>. + The numbers <em class="replaceable"><code>m</code></em> and <em class="replaceable"><code>n</code></em> within a bound are + unsigned decimal integers with permissible values from 0 to 255 inclusive. + </p><p> + <em class="firstterm">Non-greedy</em> quantifiers (available in AREs only) match the + same possibilities as their corresponding normal (<em class="firstterm">greedy</em>) + counterparts, but prefer the smallest number rather than the largest + number of matches. + See <a class="xref" href="functions-matching.html#POSIX-MATCHING-RULES" title="9.7.3.5. Regular Expression Matching Rules">Section 9.7.3.5</a> for more detail. + </p><div class="note"><h3 class="title">Note</h3><p> + A quantifier cannot immediately follow another quantifier, e.g., + <code class="literal">**</code> is invalid. + A quantifier cannot + begin an expression or subexpression or follow + <code class="literal">^</code> or <code class="literal">|</code>. + </p></div><div class="table" id="POSIX-CONSTRAINTS-TABLE"><p class="title"><strong>Table 9.19. Regular Expression Constraints</strong></p><div class="table-contents"><table class="table" summary="Regular Expression Constraints" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>Constraint</th><th>Description</th></tr></thead><tbody><tr><td> <code class="literal">^</code> </td><td> matches at the beginning of the string </td></tr><tr><td> <code class="literal">$</code> </td><td> matches at the end of the string </td></tr><tr><td> <code class="literal">(?=</code><em class="replaceable"><code>re</code></em><code class="literal">)</code> </td><td> <em class="firstterm">positive lookahead</em> matches at any point + where a substring matching <em class="replaceable"><code>re</code></em> begins + (AREs only) </td></tr><tr><td> <code class="literal">(?!</code><em class="replaceable"><code>re</code></em><code class="literal">)</code> </td><td> <em class="firstterm">negative lookahead</em> matches at any point + where no substring matching <em class="replaceable"><code>re</code></em> begins + (AREs only) </td></tr><tr><td> <code class="literal">(?<=</code><em class="replaceable"><code>re</code></em><code class="literal">)</code> </td><td> <em class="firstterm">positive lookbehind</em> matches at any point + where a substring matching <em class="replaceable"><code>re</code></em> ends + (AREs only) </td></tr><tr><td> <code class="literal">(?<!</code><em class="replaceable"><code>re</code></em><code class="literal">)</code> </td><td> <em class="firstterm">negative lookbehind</em> matches at any point + where no substring matching <em class="replaceable"><code>re</code></em> ends + (AREs only) </td></tr></tbody></table></div></div><br class="table-break" /><p> + Lookahead and lookbehind constraints cannot contain <em class="firstterm">back + references</em> (see <a class="xref" href="functions-matching.html#POSIX-ESCAPE-SEQUENCES" title="9.7.3.3. Regular Expression Escapes">Section 9.7.3.3</a>), + and all parentheses within them are considered non-capturing. + </p></div><div class="sect3" id="POSIX-BRACKET-EXPRESSIONS"><div class="titlepage"><div><div><h4 class="title">9.7.3.2. Bracket Expressions</h4></div></div></div><p> + A <em class="firstterm">bracket expression</em> is a list of + characters enclosed in <code class="literal">[]</code>. It normally matches + any single character from the list (but see below). If the list + begins with <code class="literal">^</code>, it matches any single character + <span class="emphasis"><em>not</em></span> from the rest of the list. + If two characters + in the list are separated by <code class="literal">-</code>, this is + shorthand for the full range of characters between those two + (inclusive) in the collating sequence, + e.g., <code class="literal">[0-9]</code> in <acronym class="acronym">ASCII</acronym> matches + any decimal digit. It is illegal for two ranges to share an + endpoint, e.g., <code class="literal">a-c-e</code>. Ranges are very + collating-sequence-dependent, so portable programs should avoid + relying on them. + </p><p> + To include a literal <code class="literal">]</code> in the list, make it the + first character (after <code class="literal">^</code>, if that is used). To + include a literal <code class="literal">-</code>, make it the first or last + character, or the second endpoint of a range. To use a literal + <code class="literal">-</code> as the first endpoint of a range, enclose it + in <code class="literal">[.</code> and <code class="literal">.]</code> to make it a + collating element (see below). With the exception of these characters, + some combinations using <code class="literal">[</code> + (see next paragraphs), and escapes (AREs only), all other special + characters lose their special significance within a bracket expression. + In particular, <code class="literal">\</code> is not special when following + ERE or BRE rules, though it is special (as introducing an escape) + in AREs. + </p><p> + Within a bracket expression, a collating element (a character, a + multiple-character sequence that collates as if it were a single + character, or a collating-sequence name for either) enclosed in + <code class="literal">[.</code> and <code class="literal">.]</code> stands for the + sequence of characters of that collating element. The sequence is + treated as a single element of the bracket expression's list. This + allows a bracket + expression containing a multiple-character collating element to + match more than one character, e.g., if the collating sequence + includes a <code class="literal">ch</code> collating element, then the RE + <code class="literal">[[.ch.]]*c</code> matches the first five characters of + <code class="literal">chchcc</code>. + </p><div class="note"><h3 class="title">Note</h3><p> + <span class="productname">PostgreSQL</span> currently does not support multi-character collating + elements. This information describes possible future behavior. + </p></div><p> + Within a bracket expression, a collating element enclosed in + <code class="literal">[=</code> and <code class="literal">=]</code> is an <em class="firstterm">equivalence + class</em>, standing for the sequences of characters of all collating + elements equivalent to that one, including itself. (If there are + no other equivalent collating elements, the treatment is as if the + enclosing delimiters were <code class="literal">[.</code> and + <code class="literal">.]</code>.) For example, if <code class="literal">o</code> and + <code class="literal">^</code> are the members of an equivalence class, then + <code class="literal">[[=o=]]</code>, <code class="literal">[[=^=]]</code>, and + <code class="literal">[o^]</code> are all synonymous. An equivalence class + cannot be an endpoint of a range. + </p><p> + Within a bracket expression, the name of a character class + enclosed in <code class="literal">[:</code> and <code class="literal">:]</code> stands + for the list of all characters belonging to that class. A character + class cannot be used as an endpoint of a range. + The <acronym class="acronym">POSIX</acronym> standard defines these character class + names: + <code class="literal">alnum</code> (letters and numeric digits), + <code class="literal">alpha</code> (letters), + <code class="literal">blank</code> (space and tab), + <code class="literal">cntrl</code> (control characters), + <code class="literal">digit</code> (numeric digits), + <code class="literal">graph</code> (printable characters except space), + <code class="literal">lower</code> (lower-case letters), + <code class="literal">print</code> (printable characters including space), + <code class="literal">punct</code> (punctuation), + <code class="literal">space</code> (any white space), + <code class="literal">upper</code> (upper-case letters), + and <code class="literal">xdigit</code> (hexadecimal digits). + The behavior of these standard character classes is generally + consistent across platforms for characters in the 7-bit ASCII set. + Whether a given non-ASCII character is considered to belong to one + of these classes depends on the <em class="firstterm">collation</em> + that is used for the regular-expression function or operator + (see <a class="xref" href="collation.html" title="23.2. Collation Support">Section 23.2</a>), or by default on the + database's <code class="envar">LC_CTYPE</code> locale setting (see + <a class="xref" href="locale.html" title="23.1. Locale Support">Section 23.1</a>). The classification of non-ASCII + characters can vary across platforms even in similarly-named + locales. (But the <code class="literal">C</code> locale never considers any + non-ASCII characters to belong to any of these classes.) + In addition to these standard character + classes, <span class="productname">PostgreSQL</span> defines + the <code class="literal">ascii</code> character class, which contains exactly + the 7-bit ASCII set. + </p><p> + There are two special cases of bracket expressions: the bracket + expressions <code class="literal">[[:<:]]</code> and + <code class="literal">[[:>:]]</code> are constraints, + matching empty strings at the beginning + and end of a word respectively. A word is defined as a sequence + of word characters that is neither preceded nor followed by word + characters. A word character is an <code class="literal">alnum</code> character (as + defined by the <acronym class="acronym">POSIX</acronym> character class described above) + or an underscore. This is an extension, compatible with but not + specified by <acronym class="acronym">POSIX</acronym> 1003.2, and should be used with + caution in software intended to be portable to other systems. + The constraint escapes described below are usually preferable; they + are no more standard, but are easier to type. + </p></div><div class="sect3" id="POSIX-ESCAPE-SEQUENCES"><div class="titlepage"><div><div><h4 class="title">9.7.3.3. Regular Expression Escapes</h4></div></div></div><p> + <em class="firstterm">Escapes</em> are special sequences beginning with <code class="literal">\</code> + followed by an alphanumeric character. Escapes come in several varieties: + character entry, class shorthands, constraint escapes, and back references. + A <code class="literal">\</code> followed by an alphanumeric character but not constituting + a valid escape is illegal in AREs. + In EREs, there are no escapes: outside a bracket expression, + a <code class="literal">\</code> followed by an alphanumeric character merely stands for + that character as an ordinary character, and inside a bracket expression, + <code class="literal">\</code> is an ordinary character. + (The latter is the one actual incompatibility between EREs and AREs.) + </p><p> + <em class="firstterm">Character-entry escapes</em> exist to make it easier to specify + non-printing and other inconvenient characters in REs. They are + shown in <a class="xref" href="functions-matching.html#POSIX-CHARACTER-ENTRY-ESCAPES-TABLE" title="Table 9.20. Regular Expression Character-Entry Escapes">Table 9.20</a>. + </p><p> + <em class="firstterm">Class-shorthand escapes</em> provide shorthands for certain + commonly-used character classes. They are + shown in <a class="xref" href="functions-matching.html#POSIX-CLASS-SHORTHAND-ESCAPES-TABLE" title="Table 9.21. Regular Expression Class-Shorthand Escapes">Table 9.21</a>. + </p><p> + A <em class="firstterm">constraint escape</em> is a constraint, + matching the empty string if specific conditions are met, + written as an escape. They are + shown in <a class="xref" href="functions-matching.html#POSIX-CONSTRAINT-ESCAPES-TABLE" title="Table 9.22. Regular Expression Constraint Escapes">Table 9.22</a>. + </p><p> + A <em class="firstterm">back reference</em> (<code class="literal">\</code><em class="replaceable"><code>n</code></em>) matches the + same string matched by the previous parenthesized subexpression specified + by the number <em class="replaceable"><code>n</code></em> + (see <a class="xref" href="functions-matching.html#POSIX-CONSTRAINT-BACKREF-TABLE" title="Table 9.23. Regular Expression Back References">Table 9.23</a>). For example, + <code class="literal">([bc])\1</code> matches <code class="literal">bb</code> or <code class="literal">cc</code> + but not <code class="literal">bc</code> or <code class="literal">cb</code>. + The subexpression must entirely precede the back reference in the RE. + Subexpressions are numbered in the order of their leading parentheses. + Non-capturing parentheses do not define subexpressions. + </p><div class="table" id="POSIX-CHARACTER-ENTRY-ESCAPES-TABLE"><p class="title"><strong>Table 9.20. Regular Expression Character-Entry Escapes</strong></p><div class="table-contents"><table class="table" summary="Regular Expression Character-Entry Escapes" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>Escape</th><th>Description</th></tr></thead><tbody><tr><td> <code class="literal">\a</code> </td><td> alert (bell) character, as in C </td></tr><tr><td> <code class="literal">\b</code> </td><td> backspace, as in C </td></tr><tr><td> <code class="literal">\B</code> </td><td> synonym for backslash (<code class="literal">\</code>) to help reduce the need for backslash + doubling </td></tr><tr><td> <code class="literal">\c</code><em class="replaceable"><code>X</code></em> </td><td> (where <em class="replaceable"><code>X</code></em> is any character) the character whose + low-order 5 bits are the same as those of + <em class="replaceable"><code>X</code></em>, and whose other bits are all zero </td></tr><tr><td> <code class="literal">\e</code> </td><td> the character whose collating-sequence name + is <code class="literal">ESC</code>, + or failing that, the character with octal value <code class="literal">033</code> </td></tr><tr><td> <code class="literal">\f</code> </td><td> form feed, as in C </td></tr><tr><td> <code class="literal">\n</code> </td><td> newline, as in C </td></tr><tr><td> <code class="literal">\r</code> </td><td> carriage return, as in C </td></tr><tr><td> <code class="literal">\t</code> </td><td> horizontal tab, as in C </td></tr><tr><td> <code class="literal">\u</code><em class="replaceable"><code>wxyz</code></em> </td><td> (where <em class="replaceable"><code>wxyz</code></em> is exactly four hexadecimal digits) + the character whose hexadecimal value is + <code class="literal">0x</code><em class="replaceable"><code>wxyz</code></em> + </td></tr><tr><td> <code class="literal">\U</code><em class="replaceable"><code>stuvwxyz</code></em> </td><td> (where <em class="replaceable"><code>stuvwxyz</code></em> is exactly eight hexadecimal + digits) + the character whose hexadecimal value is + <code class="literal">0x</code><em class="replaceable"><code>stuvwxyz</code></em> + </td></tr><tr><td> <code class="literal">\v</code> </td><td> vertical tab, as in C </td></tr><tr><td> <code class="literal">\x</code><em class="replaceable"><code>hhh</code></em> </td><td> (where <em class="replaceable"><code>hhh</code></em> is any sequence of hexadecimal + digits) + the character whose hexadecimal value is + <code class="literal">0x</code><em class="replaceable"><code>hhh</code></em> + (a single character no matter how many hexadecimal digits are used) + </td></tr><tr><td> <code class="literal">\0</code> </td><td> the character whose value is <code class="literal">0</code> (the null byte)</td></tr><tr><td> <code class="literal">\</code><em class="replaceable"><code>xy</code></em> </td><td> (where <em class="replaceable"><code>xy</code></em> is exactly two octal digits, + and is not a <em class="firstterm">back reference</em>) + the character whose octal value is + <code class="literal">0</code><em class="replaceable"><code>xy</code></em> </td></tr><tr><td> <code class="literal">\</code><em class="replaceable"><code>xyz</code></em> </td><td> (where <em class="replaceable"><code>xyz</code></em> is exactly three octal digits, + and is not a <em class="firstterm">back reference</em>) + the character whose octal value is + <code class="literal">0</code><em class="replaceable"><code>xyz</code></em> </td></tr></tbody></table></div></div><br class="table-break" /><p> + Hexadecimal digits are <code class="literal">0</code>-<code class="literal">9</code>, + <code class="literal">a</code>-<code class="literal">f</code>, and <code class="literal">A</code>-<code class="literal">F</code>. + Octal digits are <code class="literal">0</code>-<code class="literal">7</code>. + </p><p> + Numeric character-entry escapes specifying values outside the ASCII range + (0–127) have meanings dependent on the database encoding. When the + encoding is UTF-8, escape values are equivalent to Unicode code points, + for example <code class="literal">\u1234</code> means the character <code class="literal">U+1234</code>. + For other multibyte encodings, character-entry escapes usually just + specify the concatenation of the byte values for the character. If the + escape value does not correspond to any legal character in the database + encoding, no error will be raised, but it will never match any data. + </p><p> + The character-entry escapes are always taken as ordinary characters. + For example, <code class="literal">\135</code> is <code class="literal">]</code> in ASCII, but + <code class="literal">\135</code> does not terminate a bracket expression. + </p><div class="table" id="POSIX-CLASS-SHORTHAND-ESCAPES-TABLE"><p class="title"><strong>Table 9.21. Regular Expression Class-Shorthand Escapes</strong></p><div class="table-contents"><table class="table" summary="Regular Expression Class-Shorthand Escapes" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>Escape</th><th>Description</th></tr></thead><tbody><tr><td> <code class="literal">\d</code> </td><td> <code class="literal">[[:digit:]]</code> </td></tr><tr><td> <code class="literal">\s</code> </td><td> <code class="literal">[[:space:]]</code> </td></tr><tr><td> <code class="literal">\w</code> </td><td> <code class="literal">[[:alnum:]_]</code> + (note underscore is included) </td></tr><tr><td> <code class="literal">\D</code> </td><td> <code class="literal">[^[:digit:]]</code> </td></tr><tr><td> <code class="literal">\S</code> </td><td> <code class="literal">[^[:space:]]</code> </td></tr><tr><td> <code class="literal">\W</code> </td><td> <code class="literal">[^[:alnum:]_]</code> + (note underscore is included) </td></tr></tbody></table></div></div><br class="table-break" /><p> + Within bracket expressions, <code class="literal">\d</code>, <code class="literal">\s</code>, + and <code class="literal">\w</code> lose their outer brackets, + and <code class="literal">\D</code>, <code class="literal">\S</code>, and <code class="literal">\W</code> are illegal. + (So, for example, <code class="literal">[a-c\d]</code> is equivalent to + <code class="literal">[a-c[:digit:]]</code>. + Also, <code class="literal">[a-c\D]</code>, which is equivalent to + <code class="literal">[a-c^[:digit:]]</code>, is illegal.) + </p><div class="table" id="POSIX-CONSTRAINT-ESCAPES-TABLE"><p class="title"><strong>Table 9.22. Regular Expression Constraint Escapes</strong></p><div class="table-contents"><table class="table" summary="Regular Expression Constraint Escapes" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>Escape</th><th>Description</th></tr></thead><tbody><tr><td> <code class="literal">\A</code> </td><td> matches only at the beginning of the string + (see <a class="xref" href="functions-matching.html#POSIX-MATCHING-RULES" title="9.7.3.5. Regular Expression Matching Rules">Section 9.7.3.5</a> for how this differs from + <code class="literal">^</code>) </td></tr><tr><td> <code class="literal">\m</code> </td><td> matches only at the beginning of a word </td></tr><tr><td> <code class="literal">\M</code> </td><td> matches only at the end of a word </td></tr><tr><td> <code class="literal">\y</code> </td><td> matches only at the beginning or end of a word </td></tr><tr><td> <code class="literal">\Y</code> </td><td> matches only at a point that is not the beginning or end of a + word </td></tr><tr><td> <code class="literal">\Z</code> </td><td> matches only at the end of the string + (see <a class="xref" href="functions-matching.html#POSIX-MATCHING-RULES" title="9.7.3.5. Regular Expression Matching Rules">Section 9.7.3.5</a> for how this differs from + <code class="literal">$</code>) </td></tr></tbody></table></div></div><br class="table-break" /><p> + A word is defined as in the specification of + <code class="literal">[[:<:]]</code> and <code class="literal">[[:>:]]</code> above. + Constraint escapes are illegal within bracket expressions. + </p><div class="table" id="POSIX-CONSTRAINT-BACKREF-TABLE"><p class="title"><strong>Table 9.23. Regular Expression Back References</strong></p><div class="table-contents"><table class="table" summary="Regular Expression Back References" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>Escape</th><th>Description</th></tr></thead><tbody><tr><td> <code class="literal">\</code><em class="replaceable"><code>m</code></em> </td><td> (where <em class="replaceable"><code>m</code></em> is a nonzero digit) + a back reference to the <em class="replaceable"><code>m</code></em>'th subexpression </td></tr><tr><td> <code class="literal">\</code><em class="replaceable"><code>mnn</code></em> </td><td> (where <em class="replaceable"><code>m</code></em> is a nonzero digit, and + <em class="replaceable"><code>nn</code></em> is some more digits, and the decimal value + <em class="replaceable"><code>mnn</code></em> is not greater than the number of closing capturing + parentheses seen so far) + a back reference to the <em class="replaceable"><code>mnn</code></em>'th subexpression </td></tr></tbody></table></div></div><br class="table-break" /><div class="note"><h3 class="title">Note</h3><p> + There is an inherent ambiguity between octal character-entry + escapes and back references, which is resolved by the following heuristics, + as hinted at above. + A leading zero always indicates an octal escape. + A single non-zero digit, not followed by another digit, + is always taken as a back reference. + A multi-digit sequence not starting with a zero is taken as a back + reference if it comes after a suitable subexpression + (i.e., the number is in the legal range for a back reference), + and otherwise is taken as octal. + </p></div></div><div class="sect3" id="POSIX-METASYNTAX"><div class="titlepage"><div><div><h4 class="title">9.7.3.4. Regular Expression Metasyntax</h4></div></div></div><p> + In addition to the main syntax described above, there are some special + forms and miscellaneous syntactic facilities available. + </p><p> + An RE can begin with one of two special <em class="firstterm">director</em> prefixes. + If an RE begins with <code class="literal">***:</code>, + the rest of the RE is taken as an ARE. (This normally has no effect in + <span class="productname">PostgreSQL</span>, since REs are assumed to be AREs; + but it does have an effect if ERE or BRE mode had been specified by + the <em class="replaceable"><code>flags</code></em> parameter to a regex function.) + If an RE begins with <code class="literal">***=</code>, + the rest of the RE is taken to be a literal string, + with all characters considered ordinary characters. + </p><p> + An ARE can begin with <em class="firstterm">embedded options</em>: + a sequence <code class="literal">(?</code><em class="replaceable"><code>xyz</code></em><code class="literal">)</code> + (where <em class="replaceable"><code>xyz</code></em> is one or more alphabetic characters) + specifies options affecting the rest of the RE. + These options override any previously determined options — + in particular, they can override the case-sensitivity behavior implied by + a regex operator, or the <em class="replaceable"><code>flags</code></em> parameter to a regex + function. + The available option letters are + shown in <a class="xref" href="functions-matching.html#POSIX-EMBEDDED-OPTIONS-TABLE" title="Table 9.24. ARE Embedded-Option Letters">Table 9.24</a>. + Note that these same option letters are used in the <em class="replaceable"><code>flags</code></em> + parameters of regex functions. + </p><div class="table" id="POSIX-EMBEDDED-OPTIONS-TABLE"><p class="title"><strong>Table 9.24. ARE Embedded-Option Letters</strong></p><div class="table-contents"><table class="table" summary="ARE Embedded-Option Letters" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>Option</th><th>Description</th></tr></thead><tbody><tr><td> <code class="literal">b</code> </td><td> rest of RE is a BRE </td></tr><tr><td> <code class="literal">c</code> </td><td> case-sensitive matching (overrides operator type) </td></tr><tr><td> <code class="literal">e</code> </td><td> rest of RE is an ERE </td></tr><tr><td> <code class="literal">i</code> </td><td> case-insensitive matching (see + <a class="xref" href="functions-matching.html#POSIX-MATCHING-RULES" title="9.7.3.5. Regular Expression Matching Rules">Section 9.7.3.5</a>) (overrides operator type) </td></tr><tr><td> <code class="literal">m</code> </td><td> historical synonym for <code class="literal">n</code> </td></tr><tr><td> <code class="literal">n</code> </td><td> newline-sensitive matching (see + <a class="xref" href="functions-matching.html#POSIX-MATCHING-RULES" title="9.7.3.5. Regular Expression Matching Rules">Section 9.7.3.5</a>) </td></tr><tr><td> <code class="literal">p</code> </td><td> partial newline-sensitive matching (see + <a class="xref" href="functions-matching.html#POSIX-MATCHING-RULES" title="9.7.3.5. Regular Expression Matching Rules">Section 9.7.3.5</a>) </td></tr><tr><td> <code class="literal">q</code> </td><td> rest of RE is a literal (<span class="quote">“<span class="quote">quoted</span>”</span>) string, all ordinary + characters </td></tr><tr><td> <code class="literal">s</code> </td><td> non-newline-sensitive matching (default) </td></tr><tr><td> <code class="literal">t</code> </td><td> tight syntax (default; see below) </td></tr><tr><td> <code class="literal">w</code> </td><td> inverse partial newline-sensitive (<span class="quote">“<span class="quote">weird</span>”</span>) matching + (see <a class="xref" href="functions-matching.html#POSIX-MATCHING-RULES" title="9.7.3.5. Regular Expression Matching Rules">Section 9.7.3.5</a>) </td></tr><tr><td> <code class="literal">x</code> </td><td> expanded syntax (see below) </td></tr></tbody></table></div></div><br class="table-break" /><p> + Embedded options take effect at the <code class="literal">)</code> terminating the sequence. + They can appear only at the start of an ARE (after the + <code class="literal">***:</code> director if any). + </p><p> + In addition to the usual (<em class="firstterm">tight</em>) RE syntax, in which all + characters are significant, there is an <em class="firstterm">expanded</em> syntax, + available by specifying the embedded <code class="literal">x</code> option. + In the expanded syntax, + white-space characters in the RE are ignored, as are + all characters between a <code class="literal">#</code> + and the following newline (or the end of the RE). This + permits paragraphing and commenting a complex RE. + There are three exceptions to that basic rule: + + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + a white-space character or <code class="literal">#</code> preceded by <code class="literal">\</code> is + retained + </p></li><li class="listitem"><p> + white space or <code class="literal">#</code> within a bracket expression is retained + </p></li><li class="listitem"><p> + white space and comments cannot appear within multi-character symbols, + such as <code class="literal">(?:</code> + </p></li></ul></div><p> + + For this purpose, white-space characters are blank, tab, newline, and + any character that belongs to the <em class="replaceable"><code>space</code></em> character class. + </p><p> + Finally, in an ARE, outside bracket expressions, the sequence + <code class="literal">(?#</code><em class="replaceable"><code>ttt</code></em><code class="literal">)</code> + (where <em class="replaceable"><code>ttt</code></em> is any text not containing a <code class="literal">)</code>) + is a comment, completely ignored. + Again, this is not allowed between the characters of + multi-character symbols, like <code class="literal">(?:</code>. + Such comments are more a historical artifact than a useful facility, + and their use is deprecated; use the expanded syntax instead. + </p><p> + <span class="emphasis"><em>None</em></span> of these metasyntax extensions is available if + an initial <code class="literal">***=</code> director + has specified that the user's input be treated as a literal string + rather than as an RE. + </p></div><div class="sect3" id="POSIX-MATCHING-RULES"><div class="titlepage"><div><div><h4 class="title">9.7.3.5. Regular Expression Matching Rules</h4></div></div></div><p> + In the event that an RE could match more than one substring of a given + string, the RE matches the one starting earliest in the string. + If the RE could match more than one substring starting at that point, + either the longest possible match or the shortest possible match will + be taken, depending on whether the RE is <em class="firstterm">greedy</em> or + <em class="firstterm">non-greedy</em>. + </p><p> + Whether an RE is greedy or not is determined by the following rules: + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + Most atoms, and all constraints, have no greediness attribute (because + they cannot match variable amounts of text anyway). + </p></li><li class="listitem"><p> + Adding parentheses around an RE does not change its greediness. + </p></li><li class="listitem"><p> + A quantified atom with a fixed-repetition quantifier + (<code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">}</code> + or + <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">}?</code>) + has the same greediness (possibly none) as the atom itself. + </p></li><li class="listitem"><p> + A quantified atom with other normal quantifiers (including + <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">,</code><em class="replaceable"><code>n</code></em><code class="literal">}</code> + with <em class="replaceable"><code>m</code></em> equal to <em class="replaceable"><code>n</code></em>) + is greedy (prefers longest match). + </p></li><li class="listitem"><p> + A quantified atom with a non-greedy quantifier (including + <code class="literal">{</code><em class="replaceable"><code>m</code></em><code class="literal">,</code><em class="replaceable"><code>n</code></em><code class="literal">}?</code> + with <em class="replaceable"><code>m</code></em> equal to <em class="replaceable"><code>n</code></em>) + is non-greedy (prefers shortest match). + </p></li><li class="listitem"><p> + A branch — that is, an RE that has no top-level + <code class="literal">|</code> operator — has the same greediness as the first + quantified atom in it that has a greediness attribute. + </p></li><li class="listitem"><p> + An RE consisting of two or more branches connected by the + <code class="literal">|</code> operator is always greedy. + </p></li></ul></div><p> + </p><p> + The above rules associate greediness attributes not only with individual + quantified atoms, but with branches and entire REs that contain quantified + atoms. What that means is that the matching is done in such a way that + the branch, or whole RE, matches the longest or shortest possible + substring <span class="emphasis"><em>as a whole</em></span>. Once the length of the entire match + is determined, the part of it that matches any particular subexpression + is determined on the basis of the greediness attribute of that + subexpression, with subexpressions starting earlier in the RE taking + priority over ones starting later. + </p><p> + An example of what this means: +</p><pre class="screen"> +SELECT SUBSTRING('XY1234Z', 'Y*([0-9]{1,3})'); +<em class="lineannotation"><span class="lineannotation">Result: </span></em><code class="computeroutput">123</code> +SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})'); +<em class="lineannotation"><span class="lineannotation">Result: </span></em><code class="computeroutput">1</code> +</pre><p> + In the first case, the RE as a whole is greedy because <code class="literal">Y*</code> + is greedy. It can match beginning at the <code class="literal">Y</code>, and it matches + the longest possible string starting there, i.e., <code class="literal">Y123</code>. + The output is the parenthesized part of that, or <code class="literal">123</code>. + In the second case, the RE as a whole is non-greedy because <code class="literal">Y*?</code> + is non-greedy. It can match beginning at the <code class="literal">Y</code>, and it matches + the shortest possible string starting there, i.e., <code class="literal">Y1</code>. + The subexpression <code class="literal">[0-9]{1,3}</code> is greedy but it cannot change + the decision as to the overall match length; so it is forced to match + just <code class="literal">1</code>. + </p><p> + In short, when an RE contains both greedy and non-greedy subexpressions, + the total match length is either as long as possible or as short as + possible, according to the attribute assigned to the whole RE. The + attributes assigned to the subexpressions only affect how much of that + match they are allowed to <span class="quote">“<span class="quote">eat</span>”</span> relative to each other. + </p><p> + The quantifiers <code class="literal">{1,1}</code> and <code class="literal">{1,1}?</code> + can be used to force greediness or non-greediness, respectively, + on a subexpression or a whole RE. + This is useful when you need the whole RE to have a greediness attribute + different from what's deduced from its elements. As an example, + suppose that we are trying to separate a string containing some digits + into the digits and the parts before and after them. We might try to + do that like this: +</p><pre class="screen"> +SELECT regexp_match('abc01234xyz', '(.*)(\d+)(.*)'); +<em class="lineannotation"><span class="lineannotation">Result: </span></em><code class="computeroutput">{abc0123,4,xyz}</code> +</pre><p> + That didn't work: the first <code class="literal">.*</code> is greedy so + it <span class="quote">“<span class="quote">eats</span>”</span> as much as it can, leaving the <code class="literal">\d+</code> to + match at the last possible place, the last digit. We might try to fix + that by making it non-greedy: +</p><pre class="screen"> +SELECT regexp_match('abc01234xyz', '(.*?)(\d+)(.*)'); +<em class="lineannotation"><span class="lineannotation">Result: </span></em><code class="computeroutput">{abc,0,""}</code> +</pre><p> + That didn't work either, because now the RE as a whole is non-greedy + and so it ends the overall match as soon as possible. We can get what + we want by forcing the RE as a whole to be greedy: +</p><pre class="screen"> +SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}'); +<em class="lineannotation"><span class="lineannotation">Result: </span></em><code class="computeroutput">{abc,01234,xyz}</code> +</pre><p> + Controlling the RE's overall greediness separately from its components' + greediness allows great flexibility in handling variable-length patterns. + </p><p> + When deciding what is a longer or shorter match, + match lengths are measured in characters, not collating elements. + An empty string is considered longer than no match at all. + For example: + <code class="literal">bb*</code> + matches the three middle characters of <code class="literal">abbbc</code>; + <code class="literal">(week|wee)(night|knights)</code> + matches all ten characters of <code class="literal">weeknights</code>; + when <code class="literal">(.*).*</code> + is matched against <code class="literal">abc</code> the parenthesized subexpression + matches all three characters; and when + <code class="literal">(a*)*</code> is matched against <code class="literal">bc</code> + both the whole RE and the parenthesized + subexpression match an empty string. + </p><p> + If case-independent matching is specified, + the effect is much as if all case distinctions had vanished from the + alphabet. + When an alphabetic that exists in multiple cases appears as an + ordinary character outside a bracket expression, it is effectively + transformed into a bracket expression containing both cases, + e.g., <code class="literal">x</code> becomes <code class="literal">[xX]</code>. + When it appears inside a bracket expression, all case counterparts + of it are added to the bracket expression, e.g., + <code class="literal">[x]</code> becomes <code class="literal">[xX]</code> + and <code class="literal">[^x]</code> becomes <code class="literal">[^xX]</code>. + </p><p> + If newline-sensitive matching is specified, <code class="literal">.</code> + and bracket expressions using <code class="literal">^</code> + will never match the newline character + (so that matches will never cross newlines unless the RE + explicitly arranges it) + and <code class="literal">^</code> and <code class="literal">$</code> + will match the empty string after and before a newline + respectively, in addition to matching at beginning and end of string + respectively. + But the ARE escapes <code class="literal">\A</code> and <code class="literal">\Z</code> + continue to match beginning or end of string <span class="emphasis"><em>only</em></span>. + </p><p> + If partial newline-sensitive matching is specified, + this affects <code class="literal">.</code> and bracket expressions + as with newline-sensitive matching, but not <code class="literal">^</code> + and <code class="literal">$</code>. + </p><p> + If inverse partial newline-sensitive matching is specified, + this affects <code class="literal">^</code> and <code class="literal">$</code> + as with newline-sensitive matching, but not <code class="literal">.</code> + and bracket expressions. + This isn't very useful but is provided for symmetry. + </p></div><div class="sect3" id="POSIX-LIMITS-COMPATIBILITY"><div class="titlepage"><div><div><h4 class="title">9.7.3.6. Limits and Compatibility</h4></div></div></div><p> + No particular limit is imposed on the length of REs in this + implementation. However, + programs intended to be highly portable should not employ REs longer + than 256 bytes, + as a POSIX-compliant implementation can refuse to accept such REs. + </p><p> + The only feature of AREs that is actually incompatible with + POSIX EREs is that <code class="literal">\</code> does not lose its special + significance inside bracket expressions. + All other ARE features use syntax which is illegal or has + undefined or unspecified effects in POSIX EREs; + the <code class="literal">***</code> syntax of directors likewise is outside the POSIX + syntax for both BREs and EREs. + </p><p> + Many of the ARE extensions are borrowed from Perl, but some have + been changed to clean them up, and a few Perl extensions are not present. + Incompatibilities of note include <code class="literal">\b</code>, <code class="literal">\B</code>, + the lack of special treatment for a trailing newline, + the addition of complemented bracket expressions to the things + affected by newline-sensitive matching, + the restrictions on parentheses and back references in lookahead/lookbehind + constraints, and the longest/shortest-match (rather than first-match) + matching semantics. + </p><p> + Two significant incompatibilities exist between AREs and the ERE syntax + recognized by pre-7.4 releases of <span class="productname">PostgreSQL</span>: + + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + In AREs, <code class="literal">\</code> followed by an alphanumeric character is either + an escape or an error, while in previous releases, it was just another + way of writing the alphanumeric. + This should not be much of a problem because there was no reason to + write such a sequence in earlier releases. + </p></li><li class="listitem"><p> + In AREs, <code class="literal">\</code> remains a special character within + <code class="literal">[]</code>, so a literal <code class="literal">\</code> within a bracket + expression must be written <code class="literal">\\</code>. + </p></li></ul></div><p> + </p></div><div class="sect3" id="POSIX-BASIC-REGEXES"><div class="titlepage"><div><div><h4 class="title">9.7.3.7. Basic Regular Expressions</h4></div></div></div><p> + BREs differ from EREs in several respects. + In BREs, <code class="literal">|</code>, <code class="literal">+</code>, and <code class="literal">?</code> + are ordinary characters and there is no equivalent + for their functionality. + The delimiters for bounds are + <code class="literal">\{</code> and <code class="literal">\}</code>, + with <code class="literal">{</code> and <code class="literal">}</code> + by themselves ordinary characters. + The parentheses for nested subexpressions are + <code class="literal">\(</code> and <code class="literal">\)</code>, + with <code class="literal">(</code> and <code class="literal">)</code> by themselves ordinary characters. + <code class="literal">^</code> is an ordinary character except at the beginning of the + RE or the beginning of a parenthesized subexpression, + <code class="literal">$</code> is an ordinary character except at the end of the + RE or the end of a parenthesized subexpression, + and <code class="literal">*</code> is an ordinary character if it appears at the beginning + of the RE or the beginning of a parenthesized subexpression + (after a possible leading <code class="literal">^</code>). + Finally, single-digit back references are available, and + <code class="literal">\<</code> and <code class="literal">\></code> + are synonyms for + <code class="literal">[[:<:]]</code> and <code class="literal">[[:>:]]</code> + respectively; no other escapes are available in BREs. + </p></div><div class="sect3" id="POSIX-VS-XQUERY"><div class="titlepage"><div><div><h4 class="title">9.7.3.8. Differences from XQuery (<code class="literal">LIKE_REGEX</code>)</h4></div></div></div><a id="id-1.5.8.13.9.35.2" class="indexterm"></a><a id="id-1.5.8.13.9.35.3" class="indexterm"></a><p> + Since SQL:2008, the SQL standard includes + a <code class="literal">LIKE_REGEX</code> operator that performs pattern + matching according to the XQuery regular expression + standard. <span class="productname">PostgreSQL</span> does not yet + implement this operator, but you can get very similar behavior using + the <code class="function">regexp_match()</code> function, since XQuery + regular expressions are quite close to the ARE syntax described above. + </p><p> + Notable differences between the existing POSIX-based + regular-expression feature and XQuery regular expressions include: + + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + XQuery character class subtraction is not supported. An example of + this feature is using the following to match only English + consonants: <code class="literal">[a-z-[aeiou]]</code>. + </p></li><li class="listitem"><p> + XQuery character class shorthands <code class="literal">\c</code>, + <code class="literal">\C</code>, <code class="literal">\i</code>, + and <code class="literal">\I</code> are not supported. + </p></li><li class="listitem"><p> + XQuery character class elements + using <code class="literal">\p{UnicodeProperty}</code> or the + inverse <code class="literal">\P{UnicodeProperty}</code> are not supported. + </p></li><li class="listitem"><p> + POSIX interprets character classes such as <code class="literal">\w</code> + (see <a class="xref" href="functions-matching.html#POSIX-CLASS-SHORTHAND-ESCAPES-TABLE" title="Table 9.21. Regular Expression Class-Shorthand Escapes">Table 9.21</a>) + according to the prevailing locale (which you can control by + attaching a <code class="literal">COLLATE</code> clause to the operator or + function). XQuery specifies these classes by reference to Unicode + character properties, so equivalent behavior is obtained only with + a locale that follows the Unicode rules. + </p></li><li class="listitem"><p> + The SQL standard (not XQuery itself) attempts to cater for more + variants of <span class="quote">“<span class="quote">newline</span>”</span> than POSIX does. The + newline-sensitive matching options described above consider only + ASCII NL (<code class="literal">\n</code>) to be a newline, but SQL would have + us treat CR (<code class="literal">\r</code>), CRLF (<code class="literal">\r\n</code>) + (a Windows-style newline), and some Unicode-only characters like + LINE SEPARATOR (U+2028) as newlines as well. + Notably, <code class="literal">.</code> and <code class="literal">\s</code> should + count <code class="literal">\r\n</code> as one character not two according to + SQL. + </p></li><li class="listitem"><p> + Of the character-entry escapes described in + <a class="xref" href="functions-matching.html#POSIX-CHARACTER-ENTRY-ESCAPES-TABLE" title="Table 9.20. Regular Expression Character-Entry Escapes">Table 9.20</a>, + XQuery supports only <code class="literal">\n</code>, <code class="literal">\r</code>, + and <code class="literal">\t</code>. + </p></li><li class="listitem"><p> + XQuery does not support + the <code class="literal">[:<em class="replaceable"><code>name</code></em>:]</code> syntax + for character classes within bracket expressions. + </p></li><li class="listitem"><p> + XQuery does not have lookahead or lookbehind constraints, + nor any of the constraint escapes described in + <a class="xref" href="functions-matching.html#POSIX-CONSTRAINT-ESCAPES-TABLE" title="Table 9.22. Regular Expression Constraint Escapes">Table 9.22</a>. + </p></li><li class="listitem"><p> + The metasyntax forms described in <a class="xref" href="functions-matching.html#POSIX-METASYNTAX" title="9.7.3.4. Regular Expression Metasyntax">Section 9.7.3.4</a> + do not exist in XQuery. + </p></li><li class="listitem"><p> + The regular expression flag letters defined by XQuery are + related to but not the same as the option letters for POSIX + (<a class="xref" href="functions-matching.html#POSIX-EMBEDDED-OPTIONS-TABLE" title="Table 9.24. ARE Embedded-Option Letters">Table 9.24</a>). While the + <code class="literal">i</code> and <code class="literal">q</code> options behave the + same, others do not: + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"><p> + XQuery's <code class="literal">s</code> (allow dot to match newline) + and <code class="literal">m</code> (allow <code class="literal">^</code> + and <code class="literal">$</code> to match at newlines) flags provide + access to the same behaviors as + POSIX's <code class="literal">n</code>, <code class="literal">p</code> + and <code class="literal">w</code> flags, but they + do <span class="emphasis"><em>not</em></span> match the behavior of + POSIX's <code class="literal">s</code> and <code class="literal">m</code> flags. + Note in particular that dot-matches-newline is the default + behavior in POSIX but not XQuery. + </p></li><li class="listitem"><p> + XQuery's <code class="literal">x</code> (ignore whitespace in pattern) flag + is noticeably different from POSIX's expanded-mode flag. + POSIX's <code class="literal">x</code> flag also + allows <code class="literal">#</code> to begin a comment in the pattern, + and POSIX will not ignore a whitespace character after a + backslash. + </p></li></ul></div><p> + </p></li></ul></div><p> + </p></div></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="functions-bitstring.html" title="9.6. 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