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   pcrepattern    ( 3 )

Perl-совместимые регулярные выражения (Perl-compatible regular expressions)
  Name  |  Pcre regular expression details  |  Special start-of-pattern items  |  Ebcdic character codes  |  Characters and metacharacters  |  Backslash  |  Circumflex and dollar  |  Full stop (period, dot) and \n  |  Matching a single data unit  |  Square brackets and character classes  |  Posix character classes  |    Compatibility feature for word boundaries    |  Vertical bar  |  Internal option setting  |  Subpatterns  |  Duplicate subpattern numbers  |  Named subpatterns  |  Repetition  |  Atomic grouping and possessive quantifiers  |  Back references  |  Assertions  |  Conditional subpatterns  |  Comments  |  Recursive patterns  |  Subpatterns as subroutines  |  Oniguruma subroutine syntax  |  Callouts  |  Backtracking control  |  See also  |

COMPATIBILITY FEATURE FOR WORD BOUNDARIES

In the POSIX.2 compliant library that was included in 4.4BSD
       Unix, the ugly syntax [[:<:]] and [[:>:]] is used for matching
       "start of word" and "end of word". PCRE treats these items as
       follows:


         [[:<:]]  is converted to  \b(?=\w)
         [[:>:]]  is converted to  \b(?<=\w)


       Only these exact character sequences are recognized. A sequence
       such as [a[:<:]b] provokes error for an unrecognized POSIX class
       name. This support is not compatible with Perl. It is provided to
       help migrations from other environments, and is best not used in
       any new patterns. Note that \b matches at the start and the end
       of a word (see "Simple assertions" above), and in a Perl-style
       pattern the preceding or following character normally shows which
       is wanted, without the need for the assertions that are used
       above in order to give exactly the POSIX behaviour.