From: Peter Heslin <[EMAIL PROTECTED]>
Sent: Thursday, August 31, 2000 10:49 PM
> Simply put, I want variable-length lookbehind.
The RFC seems to say you want this so that you can optimize the operation of
the regex execution. I've been looking at the existing fixed-length
look-behind and see that it does not seem to operate the way you may have
assumed.
In particular, it is greedy in the sense of the forward matching "*" or "+"
constructs.
Here is a demonstation with the regex-debugging switch enabled. Note that
the look-behind test fails 7 times, before succeeding on the 8th try. If
the "*" was greedier than the look-behind, you would expect 2 look-behind
failures, and success on the 3rd.
This doesn't argue against a variable-length lookbehind as a useful feature,
but it may not be what you were looking for.
- mike mulligan
$ perl "-Dr" -e "print qq($&\n) if qq(mxnxxxxoxxx) =~ /(?<=[aeiou])x*/"
Compiling REx `(?<=[aeiou])x*'
size 17 first at 1
1: IFMATCH[-1](14)
3: ANYOF[aeiou](12)
12: SUCCEED(0)
13: TAIL(14)
14: STAR(17)
15: EXACT <x>(0)
17: END(0)
minlen 0
Enabling $` $& $' support.
EXECUTING...
Matching REx `(?<=[aeiou])x*' against `mxnxxxxoxxx'
Setting an EVAL scope, savestack=3
0 <> <mxnxxxxoxxx> | 1: IFMATCH[-1]
failed...
Setting an EVAL scope, savestack=3
1 <m> <xnxxxxoxxx> | 1: IFMATCH[-1]
0 <> <mxnxxxxoxxx> | 3: ANYOF[aeiou]
failed...
failed...
Setting an EVAL scope, savestack=3
2 <mx> <nxxxxoxxx> | 1: IFMATCH[-1]
1 <m> <xnxxxxoxxx> | 3: ANYOF[aeiou]
failed...
failed...
Setting an EVAL scope, savestack=3
3 <mxn> <xxxxoxxx> | 1: IFMATCH[-1]
2 <mx> <nxxxxoxxx> | 3: ANYOF[aeiou]
failed...
failed...
Setting an EVAL scope, savestack=3
4 <mxnx> <xxxoxxx> | 1: IFMATCH[-1]
3 <mxn> <xxxxoxxx> | 3: ANYOF[aeiou]
failed...
failed...
Setting an EVAL scope, savestack=3
5 <mxnxx> <xxoxxx> | 1: IFMATCH[-1]
4 <mxnx> <xxxoxxx> | 3: ANYOF[aeiou]
failed...
failed...
Setting an EVAL scope, savestack=3
6 <mxnxxx> <xoxxx> | 1: IFMATCH[-1]
5 <mxnxx> <xxoxxx> | 3: ANYOF[aeiou]
failed...
failed...
Setting an EVAL scope, savestack=3
7 <mxnxxxx> <oxxx> | 1: IFMATCH[-1]
6 <mxnxxx> <xoxxx> | 3: ANYOF[aeiou]
failed...
failed...
Setting an EVAL scope, savestack=3
8 <mxnxxxxo> <xxx> | 1: IFMATCH[-1]
7 <mxnxxxx> <oxxx> | 3: ANYOF[aeiou]
8 <mxnxxxxo> <xxx> | 12: SUCCEED
could match...
8 <mxnxxxxo> <xxx> | 14: STAR
EXACT <x> can match 3 times out of 32767...
Setting an EVAL scope, savestack=3
11 <mxnxxxxoxxx> <> | 17: END
Match successful!
xxx
Freeing REx: `(?<=[aeiou])x*'
