Charles Levert <[EMAIL PROTECTED]> writes: > * On Sunday 2005-11-06 at 23:51:12 +0100, Andreas Schwab wrote: >> Charles Levert <[EMAIL PROTECTED]> writes: >> >> > * On Sunday 2005-11-06 at 22:48:30 +0100, Andreas Schwab wrote: >> >> only in BREs which don't have alternation. >> > >> > Is that the justification? >> >> I don't understand your question, but my point is that if you don't have >> alternation (and neither optional matches) the group referenced by a >> back-reference has always matched something fixed. > > Well, no, as my example showed. It contained > a bracket expression, [xy], and these are > not fixed, are allowed in BREs, and are the > equivalent of an alternation like (x|y).
No, they are not equivalent. You can back-references only a complete bracket expression (and when you reference it it has matched a fixed string), whereas an alternation has sub expressions of its own. Thus with brackets the issue of back-referencing a sub expression that didn't match cannot occur. Andreas. -- Andreas Schwab, SuSE Labs, [EMAIL PROTECTED] SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany PGP key fingerprint = 58CA 54C7 6D53 942B 1756 01D3 44D5 214B 8276 4ED5 "And now for something completely different."
