* On Sunday 2005-11-06 at 22:48:30 +0100, Andreas Schwab wrote: > Note that POSIX does not allow back-references in EREs,
You're right. I didn't notice that because GNU grep does support them (from GNU regex). > only in BREs which don't have alternation. Is that the justification? Strictly speaking, yes, '([xy])z\1' can always be replaced by 'xzx|yzy', but with more complicated cases, back-references can seriously cut down the complexity of a regular expression.
