grep: Basic vs Extended
3.6 Basic vs Extended Regular Expressions
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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.
Info Catalog
grep: Back-references and Subexpressions
grep: Regular Expressions