File: grep.info,  Node: grep Programs,  Prev: Exit Status,  Up: Invoking

2.4 ‘grep’ Programs
===================

‘grep’ searches the named input files for lines containing a match to
the given patterns.  By default, ‘grep’ prints the matching lines.  A
file named ‘-’ stands for standard input.  If no input is specified,
‘grep’ searches the working directory ‘.’ if given a command-line option
specifying recursion; otherwise, ‘grep’ searches standard input.  There
are four major variants of ‘grep’, controlled by the following options.

‘-G’
‘--basic-regexp’
     Interpret patterns as basic regular expressions (BREs).  This is
     the default.

‘-E’
‘--extended-regexp’
     Interpret patterns as extended regular expressions (EREs).  (‘-E’
     is specified by POSIX.)

‘-F’
‘--fixed-strings’
     Interpret patterns as fixed strings, not regular expressions.
     (‘-F’ is specified by POSIX.)

‘-P’
‘--perl-regexp’
     Interpret patterns as Perl-compatible regular expressions (PCREs).
     PCRE support is here to stay, but consider this option experimental
     when combined with the ‘-z’ (‘--null-data’) option, and note that
     ‘grep -P’ may warn of unimplemented features.  *Note Other
     Options::.

     For documentation, refer to , with these
     caveats:
        • ‘\d’ matches only the ten ASCII digits (and ‘\D’ matches the
          complement), regardless of locale.  Use ‘\p{Nd}’ to also match
          non-ASCII digits.  (The behavior of ‘\d’ and ‘\D’ is
          unspecified after in-regexp directives like ‘(?aD)’.)

        • Although PCRE tracks the syntax and semantics of Perl's
          regular expressions, the match is not always exact.  For
          example, Perl evolves and a Perl installation may predate or
          postdate the PCRE2 installation on the same host, or their
          Unicode versions may differ, or Perl and PCRE2 may disagree
          about an obscure construct.

        • By default, ‘grep’ applies each regexp to a line at a time, so
          the ‘(?s)’ directive (making ‘.’ match line breaks) is
          generally ineffective.  However, with ‘-z’ (‘--null-data’) it
          can work:
               $ printf 'a\nb\n' |grep -zP '(?s)a.b'
               a
               b
          But beware: with the ‘-z’ (‘--null-data’) and a file
          containing no NUL byte, grep must read the entire file into
          memory before processing any of it.  Thus, it will exhaust
          memory and fail for some large files.