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For complete details please consult the man pages for PCRE, especially
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For complete details please consult the man pages for PCRE, especially
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\code{man pcrepattern} and \code{man pcreapi}) on your system or from
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\code{man pcrepattern} and \code{man pcreapi}) on your system or from
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the sources at
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the sources at
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\url{ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/}. If PCRE
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\url{ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/}. If PCRE
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support was compiled from the sources within \R, the PCRE version is 4.4
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support was compiled from the sources within \R, the PCRE version is 4.5
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as described here (version \eqn{\ge}{>=} 4.0 is required even if \R is
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as described here (version \eqn{\ge}{>=} 4.0 is required even if \R is
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configured to use the system's PCRE library).
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configured to use the system's PCRE library).
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All the regular expressions described for extended regular expressions
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All the regular expressions described for extended regular expressions
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are accepted except \code{\\<} and \code{\\>}: in Perl all backslashed
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are accepted except \code{\\<} and \code{\\>}: in Perl all backslashed
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metacharacters are alphanumeric and backslashed symbols always are
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metacharacters are alphanumeric and backslashed symbols always are
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interpreted as a literal character. \code{\{} is not special if it would
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interpreted as a literal character. \code{\{} is not special if it would
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be the start of an invalid interval specification. There can be more than
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be the start of an invalid interval specification. There can be more than
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9 backreferences.
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9 backreferences.
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The construct \code{(?...)} is used for Perl extensions in a variety
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The construct \code{(?...)} is used for Perl extensions in a variety
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of ways depending on what immediately follows the \code{?}.
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of ways depending on what immediately follows the \code{?}.
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\code{cntrl-x} for any \code{x}, \code{\\ddd} is the octal character
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\code{cntrl-x} for any \code{x}, \code{\\ddd} is the octal character
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\code{ddd} (for up to three digits unless interpretable as a
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\code{ddd} (for up to three digits unless interpretable as a
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backreference), and \code{\\xhh} specifies a character in hex.
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backreference), and \code{\\xhh} specifies a character in hex.
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Outside a character class, \code{\\b} matches a word boundary,
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Outside a character class, \code{\\b} matches a word boundary,
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\code{\\B} is its negation, \code{\\A} matches at start of subject (even
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\code{\\B} is its negation, \code{\\A} matches at start of a subject (even
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in multiline mode, unlike \code{^}), \code{\\Z} matches at end of a
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in multiline mode, unlike \code{^}), \code{\\Z} matches at end of a
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subject or before newline at end, \code{\\z} matches at end of a
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subject or before newline at end, \code{\\z} matches at end of a
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subject. and \code{\\G} matches at first matching position in a
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subject. and \code{\\G} matches at first matching position in a
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subject. \code{\\C} matches a single byte. including a newline.
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subject. \code{\\C} matches a single byte. including a newline.
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