Exim uses the PCRE regular expression library; this provides regular expression matching that is compatible with Perl 5. The format of these regular expressions is described in many Perl reference books, and also in Jeffrey Friedl's Mastering Regular Expressions (O'Reilly, ISBN 1-56592-257-3).
The PCRE distribution files, which are included in the directory `src/pcre' in the Exim distribution, contain a man page for PCRE which describes exactly what it supports, so no further description is included here. The PCRE functions are called from Exim using the default option settings, except when processing the `matches' action in an Exim filter, where PCRE_CASELESS is set to cause matching to be independent of the case of letters.
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