Lookbehind Regex

[GoogleCodeExporter]

PROBLEM #1
Lookbehind eg (?<=x) is not accepted as a regular expression, even though
Lookahead eg (?=\x)is accepted

PROBLEM #2
In fact FoxReplace erroneously treats Lookahead as if it were Lookbehind.
Mind you I am new to Lookaround so I may not be understanding. I am using the 
following as a reference/test http://gskinner.com/RegExr/ (so Skinner may be 
wrong not FoxReplace).

Original issue reported on code.google.com by sqgl2...@gmail.com on 21 Apr 2013 at 6:24

[GoogleCodeExporter]

Regular expressions in FoxReplace are regular JavaScript expressions, so only 
what is supported by JavaScript can be implemented. There is also some use of 
XRegExp, which allows some partial emulation of lookbehind, but according to 
the author it is limited: 
http://blog.stevenlevithan.com/archives/javascript-regex-lookbehind

As for problem #2, it should work correctly, as the regexp is passed directly 
to the constructor of RegExp. Do you have some specific example that shows this 
wrong behaviour?

Original comment by marc.r...@gmail.com on 22 Apr 2013 at 7:41

• Added labels: OpSys-All
• Removed labels: OpSys-Windows

[GoogleCodeExporter]

----------------------
EXAMPLE FOR PROBLEM #2
----------------------
REPLACE
(?=4)(20)
WITH
twenty

So if I apply this to
420
Foxreplace will convert it to
4twenty
but it should not do anything (I think, since I do not really understand 
LookAhead)

So FoxReplace acts as if I has used (?<=4), ie LookBehind, rather than (?=4), 
ie LookAhead

And indeed, Foxreplace will not convert
520
to anything other than
520

Original comment by sqgl2...@gmail.com on 23 Apr 2013 at 3:08

[GoogleCodeExporter]

I have tried to reproduce your example on this very page and it doesn't replace 
anything (tested on FoxReplace 0.14.0 and Firefox 21.0).

Original comment by marc.r...@gmail.com on 24 Apr 2013 at 3:15

[GoogleCodeExporter]

I was wrong about PROBLEM #2. Sorry, I cannot reproduce it either (no I wasn't 
stoned).

PROBLEM #1 Remains. Mind you I don't know why I used a backslash in my example 
so let me restate it thus:

PROBLEM #1
Lookbehind eg (?<=x) is not accepted as a regular expression, even though
Lookahead eg (?=x)is accepted

I thought it might be because it requires "extended" regex but on Skinner's 
test page only the Lookahead requires turning on the "extended" option, so 
Lookbehind should work regardless (it appears the JavaScript being employed by 
FoxReplace is using the "extended" option anyhow, since Lookahead works).

Original comment by sqgl2...@gmail.com on 16 Jul 2013 at 2:44

[GoogleCodeExporter]

Ok, now I have researched some more on the problem.

That test page, RegExr, is implemented in Flex, according to the author, and so 
it's programmed in ActionScript. Although both ActionScript and JavaScript are 
implementations of the same ECMA-262 standard, they have some differences. One 
of the differences is that ActionScript (and thus RegExr) supports lookbehind 
but JavaScript (and thus Firefox and thus FoxReplace) doesn't; the ECMA-262 
standard doesn't support lookbehind either (source: 
http://www.regular-expressions.info/refflavors.html), so it's an added feature 
of ActionScript because of the regex engine it uses (source: 
http://stackoverflow.com/a/6441148/792577).

So, the only way to support lookbehind is to wait until some day Firefox 
supports it, if ever (then they would be supported in FoxReplace 
automatically). Or maybe trying to integrate a third party regex engine, with 
all the potential issues (compatibility, portability, licenses, new bugs, etc.) 
this could create.

I don't want to mess with alternative regex implementations, so I will close 
the issue because there isn't a good solution, sorry.

Original comment by marc.r...@gmail.com on 16 Jul 2013 at 4:08

• Changed title: Lookbehind Regex
• Changed state: WontFix
• Added labels: Type-Enhancement
• Removed labels: Type-Defect