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# Regular Expression

A regular expression is a string that specifies a pattern that is used to search text. You can perform a search for single or multiple matches, or extract the matched text from the string. You can also merge new text in to replace the matches, or apply a verb to the matched text in a string.
Utility verbs are included to assist in building regular expression patterns, as is a utility to search J scripts and other text files for patterns.
The primary definitions are in system\main\regex.ijs ; utility verbs to build patterns are in system\packages\regex\regbuild.ijs .
The Find in Files utility, available from the Edit menu or by pressing Ctrl+Shift+F, can search for simple text, regular expression, or special patterns such as the assignment of a name.
Labs and a demo on regular expressions are available from the Studio menu.
J uses the PCRE (Perl Compatible Regular Expression) engine through the POSIX regex interface. The complete PCRE documentation is available, though the next pages here are probably adequate for most applications. The PCRE library is covered by a University of Cambridge copyright.

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