add pcre to in tree libs

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@3732 d0543943-73ff-0310-b7d9-9358b9ac24b2

libs/pcre/doc/pcrecompat.3

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+.TH PCRECOMPAT 3
+.SH NAME
+PCRE - Perl-compatible regular expressions
+.SH "DIFFERENCES BETWEEN PCRE AND PERL"
+.rs
+.sp
+This document describes the differences in the ways that PCRE and Perl handle
+regular expressions. The differences described here are with respect to Perl
+5.8.
+.P
+1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
+it does have are given in the
+.\" HTML <a href="pcre.html#utf8support">
+.\" </a>
+section on UTF-8 support
+.\"
+in the main
+.\" HREF
+\fBpcre\fP
+.\"
+page.
+.P
+2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
+them, but they do not mean what you might think. For example, (?!a){3} does
+not assert that the next three characters are not "a". It just asserts that the
+next character is not "a" three times.
+.P
+3. Capturing subpatterns that occur inside negative lookahead assertions are
+counted, but their entries in the offsets vector are never set. Perl sets its
+numerical variables from any such patterns that are matched before the
+assertion fails to match something (thereby succeeding), but only if the
+negative lookahead assertion contains just one branch.
+.P
+4. Though binary zero characters are supported in the subject string, they are
+not allowed in a pattern string because it is passed as a normal C string,
+terminated by zero. The escape sequence \e0 can be used in the pattern to
+represent a binary zero.
+.P
+5. The following Perl escape sequences are not supported: \el, \eu, \eL,
+\eU, and \eN. In fact these are implemented by Perl's general string-handling
+and are not part of its pattern matching engine. If any of these are
+encountered by PCRE, an error is generated.
+.P
+6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
+built with Unicode character property support. The properties that can be
+tested with \ep and \eP are limited to the general category properties such as
+Lu and Nd, script names such as Greek or Han, and the derived properties Any
+and L&.
+.P
+7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
+between are treated as literals. This is slightly different from Perl in that $
+and @ are also handled as literals inside the quotes. In Perl, they cause
+variable interpolation (but of course PCRE does not have variables). Note the
+following examples:
+.sp
+    Pattern            PCRE matches      Perl matches
+.sp
+.\" JOIN
+    \eQabc$xyz\eE        abc$xyz           abc followed by the
+                                           contents of $xyz
+    \eQabc\e$xyz\eE       abc\e$xyz          abc\e$xyz
+    \eQabc\eE\e$\eQxyz\eE   abc$xyz           abc$xyz
+.sp
+The \eQ...\eE sequence is recognized both inside and outside character classes.
+.P
+8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
+constructions. However, there is support for recursive patterns using the
+non-Perl items (?R), (?number), and (?P>name). Also, the PCRE "callout" feature
+allows an external function to be called during pattern matching. See the
+.\" HREF
+\fBpcrecallout\fP
+.\"
+documentation for details.
+.P
+9. There are some differences that are concerned with the settings of captured
+strings when part of a pattern is repeated. For example, matching "aba" against
+the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
+.P
+10. PCRE provides some extensions to the Perl regular expression facilities:
+.sp
+(a) Although lookbehind assertions must match fixed length strings, each
+alternative branch of a lookbehind assertion can match a different length of
+string. Perl requires them all to have the same length.
+.sp
+(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
+meta-character matches only at the very end of the string.
+.sp
+(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
+meaning is faulted. Otherwise, like Perl, the backslash is ignored. (Perl can
+be made to issue a warning.)
+.sp
+(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
+inverted, that is, by default they are not greedy, but if followed by a
+question mark they are.
+.sp
+(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
+only at the first matching position in the subject string.
+.sp
+(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
+options for \fBpcre_exec()\fP have no Perl equivalents.
+.sp
+(g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern
+matching (Perl can do this using the (?p{code}) construct, which PCRE cannot
+support.)
+.sp
+(h) PCRE supports named capturing substrings, using the Python syntax.
+.sp
+(i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java
+package.
+.sp
+(j) The (R) condition, for testing recursion, is a PCRE extension.
+.sp
+(k) The callout facility is PCRE-specific.
+.sp
+(l) The partial matching facility is PCRE-specific.
+.sp
+(m) Patterns compiled by PCRE can be saved and re-used at a later time, even on
+different hosts that have the other endianness.
+.sp
+(n) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
+different way and is not Perl-compatible.
+.P
+.in 0
+Last updated: 06 June 2006
+.br
+Copyright (c) 1997-2006 University of Cambridge.