grep.info: Basic vs Extended

Regular Expressions

Back-references and Subexpressions

Back to Software Index

3.6 Basic vs Extended Regular Expressions
=========================================

In basic regular expressions the meta-characters `?', `+', `{', `|',
`(', and `)' lose their special meaning; instead use the backslashed
versions `\?', `\+', `\{', `\|', `\(', and `\)'.

   Traditional `egrep' did not support the `{' meta-character, and some
`egrep' implementations support `\{' instead, so portable scripts
should avoid `{' in `grep -E' patterns and should use `[{]' to match a
literal `{'.

   GNU `grep -E' attempts to support traditional usage by assuming that
`{' is not special if it would be the start of an invalid interval
specification.  For example, the command `grep -E '{1'' searches for
the two-character string `{1' instead of reporting a syntax error in
the regular expression.  POSIX allows this behavior as an extension,
but portable scripts should avoid it.