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In basic regular expressions the metacharacters `?', `+', `{', `|', `(', and `)' lose their special meaning; instead use the backslashed versions `\?', `\+', `\{', `\|', `\(', and `\)'.
Traditional egrep did not support the `{' metacharacter,
and some egrep implementations support `\{' instead, so
portable scripts should avoid `{' in `egrep' patterns and
should use `[{]' to match a literal `{'.
GNU egrep 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 shell command
`egrep '{1'' searches for the two-character string `{1'
instead of reporting a syntax error in the regular expression.
POSIX.2 allows this behavior as an extension, but portable scripts
should avoid it.