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Regular Expression Dialects

Regular expressions have evolved over the years, and the various tools that handle regular expressions have different capabilities and slightly different syntax.

In particular, the original Unix search tool grep came in three varieties:

  • fgrep, which could search only for fixed text patterns
  • grep, which handled basic regular expressions
  • egrep, which handled an extended form of regular expressions

The GNU project originally shipped all three commands, but fgrep and egrep were never fully standardized, so they were removed from the Posix standard in 2001. They were recently also removed from the GNU project.

Unlike the original Unix grep, the GNU grep can handle the full extended regular expression syntax, in either of two ways:

  • To use the special characters (called "meta-characters") preceed them with a backslash. In other words, while a backslash makes special characters like . or ordinary, it also makes ordinary characters into special characters.

  • Alternately, use the -E option to make grep understand extended regular expressions, which causes metacharacters to become special characters.

Other tools, such as sed, similarly require backslashes in front of some of the extended regexp meta-characters (or, if you're using a GNU version of sed, you can use the -E option to enable extended regular expressions, just like GNU grep).

The Perl language introduced one of the most powerful and consistent versions of the regular expression language. There has been increasing consensus around "Perl-Compatible Regular Extensions" (aka PCRE) and that dialect is available in many tools (including GNU grep via the -P option, as well as the PCRE/PCRE2 library for C and C++ programs, which is used in many software packages including Safari and Apache httpd).