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List of regular expression libraries Name Official website Programming language Software license Used by Boost.Regex [Note 1] Boost C++ Libraries: C++: Boost: Notepad++ >= 6.0.0, EmEditor: Boost.Xpressive Boost C++ Libraries: C++ Boost DEELX RegExLab: C++ Proprietary FREJ [Note 2] Fuzzy Regular Expressions for Java: Java: LGPL GLib/GRegex [Note 3]
More generally, an equation E=F between regular-expression terms with variables holds if, and only if, its instantiation with different variables replaced by different symbol constants holds. [30] [31] Every regular expression can be written solely in terms of the Kleene star and set unions over finite words. This is a surprisingly difficult ...
In computer science, Thompson's construction algorithm, also called the McNaughton–Yamada–Thompson algorithm, [1] is a method of transforming a regular expression into an equivalent nondeterministic finite automaton (NFA). [2] This NFA can be used to match strings against the regular expression.
The simplest pattern in pattern matching is an explicit value or a variable. For an example, consider a simple function definition in Haskell syntax (function parameters are not in parentheses but are separated by spaces, = is not assignment but definition):
There are several code generation options; normally re2c uses switch statements, but it can use nested if statements (as in this example with -s option), or generate bitmaps and jump tables. Which option is better depends on the C compiler; re2c users are encouraged to experiment.
If the portion of the regular expression is "greedy", it will match as many characters as possible. If it is not greedy, it will match as few characters as possible. By default, quantifiers in AWB are greedy. To make a quantifier non-greedy, it must be followed by a question mark. For example: In this string:
Extreme examples occur when these are combined – Uniform Naming Convention paths begin with \\, and thus an escaped regular expression matching a UNC name begins with 8 backslashes, "\\\\\", due to needing to escape the string and the regular expression. Using raw strings reduces this to 4 (escaping in the regular expression), as in C# @"\\\\".
The pcregrep command is an implementation of grep that uses Perl regular expression syntax. [17] Similar functionality can be invoked in the GNU version of grep with the -P flag. [18] Ports of grep (within Cygwin and GnuWin32, for example) also run under Microsoft Windows. Some versions of Windows feature the similar qgrep or findstr command. [19]