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diff --git a/examples/prompts/regular-language.py b/examples/prompts/regular-language.py
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+++ b/examples/prompts/regular-language.py
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+#!/usr/bin/env python
+"""
+This is an example of "prompt_toolkit.contrib.regular_languages" which
+implements a little calculator.
+
+Type for instance::
+
+    > add 4 4
+    > sub 4 4
+    > sin 3.14
+
+This example shows how you can define the grammar of a regular language and how
+to use variables in this grammar with completers and tokens attached.
+"""
+import math
+
+from prompt_toolkit import prompt
+from prompt_toolkit.completion import WordCompleter
+from prompt_toolkit.contrib.regular_languages.compiler import compile
+from prompt_toolkit.contrib.regular_languages.completion import GrammarCompleter
+from prompt_toolkit.contrib.regular_languages.lexer import GrammarLexer
+from prompt_toolkit.lexers import SimpleLexer
+from prompt_toolkit.styles import Style
+
+operators1 = ["add", "sub", "div", "mul"]
+operators2 = ["cos", "sin"]
+
+
+def create_grammar():
+    return compile(
+        r"""
+        (\s*  (?P<operator1>[a-z]+)   \s+   (?P<var1>[0-9.]+)   \s+   (?P<var2>[0-9.]+)   \s*) |
+        (\s*  (?P<operator2>[a-z]+)   \s+   (?P<var1>[0-9.]+)   \s*)
+    """
+    )
+
+
+example_style = Style.from_dict(
+    {
+        "operator": "#33aa33 bold",
+        "number": "#ff0000 bold",
+        "trailing-input": "bg:#662222 #ffffff",
+    }
+)
+
+
+if __name__ == "__main__":
+    g = create_grammar()
+
+    lexer = GrammarLexer(
+        g,
+        lexers={
+            "operator1": SimpleLexer("class:operator"),
+            "operator2": SimpleLexer("class:operator"),
+            "var1": SimpleLexer("class:number"),
+            "var2": SimpleLexer("class:number"),
+        },
+    )
+
+    completer = GrammarCompleter(
+        g,
+        {
+            "operator1": WordCompleter(operators1),
+            "operator2": WordCompleter(operators2),
+        },
+    )
+
+    try:
+        # REPL loop.
+        while True:
+            # Read input and parse the result.
+            text = prompt(
+                "Calculate: ", lexer=lexer, completer=completer, style=example_style
+            )
+            m = g.match(text)
+            if m:
+                vars = m.variables()
+            else:
+                print("Invalid command\n")
+                continue
+
+            print(vars)
+            if vars.get("operator1") or vars.get("operator2"):
+                try:
+                    var1 = float(vars.get("var1", 0))
+                    var2 = float(vars.get("var2", 0))
+                except ValueError:
+                    print("Invalid command (2)\n")
+                    continue
+
+                # Turn the operator string into a function.
+                operator = {
+                    "add": (lambda a, b: a + b),
+                    "sub": (lambda a, b: a - b),
+                    "mul": (lambda a, b: a * b),
+                    "div": (lambda a, b: a / b),
+                    "sin": (lambda a, b: math.sin(a)),
+                    "cos": (lambda a, b: math.cos(a)),
+                }[vars.get("operator1") or vars.get("operator2")]
+
+                # Execute and print the result.
+                print("Result: %s\n" % (operator(var1, var2)))
+
+            elif vars.get("operator2"):
+                print("Operator 2")
+
+    except EOFError:
+        pass