regular expression addition - lookbehind assertions and lookahead assertions

[liamquin]

regular expression addition - lookbehind assertions and lookahead assertions

[michaelhkay]

This sounds feasible to me, though it's a fair bit of work (on specification, tests, and implementation). I suggest we restrict it so that (a) lookbehind has to be fixed length, and (b) neither lookahead nor lookbehind allows capturing groups.

Lookbehind syntax uses the < character which will make it rather inconvenient, especially as &-escaping inside string literals is recognized (and required) in XQuery but not in XPath. Perhaps we should allow leftwards arrow (U+2190) as an alternative.

[liamquin]

We could use the *plb *pla *nlb *nla syntax instead of < maybe, or even the longer versions ("terseness shall not be a goal" hah).

The restrictions are OK if sometimes a pain; i think libpcre has them too.

You’ve been wanting to use non-ASCII characters for yonks, now’s the chance :-) but i’d rather use something people have a chance of looking up online, so i'd go for (*posivitve-lookbehind-assertion:foo) i think.

People already have trouble with { } in regular expressions, it’s an excellent point.