From: Hugo <[EMAIL PROTECTED]>
Sent: Monday, September 11, 2000 11:59 PM
> mike mulligan replied to Peter Heslin:
> : ... it is greedy in the sense of the forward matching "*" or "+"
constructs.
> : [snip]
>
> This is nothing to do with greediness and everything to do with
> left-to-rightness. The regexp engine does not look for x* except
> in those positions where the lookbehind has already matched.
I was trying to understand at what point the lookbehind was attempted, and
confused myself and posted a bad example. My apologies to everyone. Let's
see if I can make sense of it on a second try.
My question is: if I have the regex /(?<=[aeiou]X[yz]+/ then does Perl: 1.
scan first for 'X', test the lookbehind, and then test the '[yz]', or 2.
scan for 'X[yz]' and then test the lookbehind?
I am expecting these two alternatives to give the same result, but certain
test strings might run slower or faster depending on the approach.
Running perl -Dr shows that alternative 1 is used:
qq(aXuhXvoXyz) =~ /(?<=[aeiou])X[yz]/
Guessing start of match, REx `(?<=[aeiou])X[yz]' against `aXuhXvoXyz'...
Found anchored substr `X' at offset 1...
Guessed: match at offset 1
Matching REx `(?<=[aeiou])X[yz]' against `XuhXvoXyz'
Setting an EVAL scope, savestack=3
1 <a> <XuhXvoXyz> | 1: IFMATCH[-1]
0 <> <aXuhXvoXyz> | 3: ANYOF[aeiou]
1 <a> <XuhXvoXyz> | 12: SUCCEED
could match...
1 <a> <XuhXvoXyz> | 14: EXACT <X>
2 <aX> <uhXvoXyz> | 16: ANYOF[yz]
failed...
Setting an EVAL scope, savestack=3
4 <aXuh> <XvoXyz> | 1: IFMATCH[-1]
3 <aXu> <hXvoXyz> | 3: ANYOF[aeiou]
failed...
failed...
Setting an EVAL scope, savestack=3
7 <aXuhXvo> <Xyz> | 1: IFMATCH[-1]
6 <aXuhXv> <oXyz> | 3: ANYOF[aeiou]
7 <aXuhXvo> <Xyz> | 12: SUCCEED
could match...
7 <aXuhXvo> <Xyz> | 14: EXACT <X>
8 <aXuhXvoX> <yz> | 16: ANYOF[yz]
9 <aXuhXvoXy> <z> | 25: END
Match successful!
