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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. See Other Options.

For documentation, refer to https://www.pcre.org/, 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.