Extended regexes - "(?<...)" construct.

[Cloink]

Can anyone tell me why my version of Perl won't compile a so-called
positive lookbehind assertion.

Specifically, I want to strip JavaScript comments from a file, so I'm
looking for "//" on a line, but since JS code can use regexes too,
it's possible to have "//" in a js-regex, which clearly isn't a
comment.

If the "//" is preceded by backslash however, i.e. "\//", I can be
pretty sure that this does NOT represent a comment. (I don't need
advice on when this rule might break - I have a very specific
purpose.)

Here's one piece of problematic (javascript) code
.split(/\//); // split string at forward-slash delimiters.

My PERL regex, in attempting to identify a preceding backslash in the
js code, I WANT to look like this
s/(?<[^\\])\/\/.*$//x
to identify two slashes on a line but not immediately preceded by a
backslash.

But the perl compiler tells me it doesn't understand:-
Sequence (?<[...) not recognized in regex; [line cont..]
marked by <-- HERE in m/(?<[ <-- HERE ^\\])//.*$/

Incidentally, if I replace the awkward "not-a-backslash" section of
the regex with a more simple "an x" section, I still get the same
compilation error, i.e.
replace "[^\\]" with "x"

"perl -v" tells me
This is perl, v5.8.8 built for MSWin32-x86-multi-thread
and
Binary build 820 [274739] provided by ActiveState
http://www.ActiveState.com
Built Jan 23 2007 15:57:46

It's not my reference book that's wrong is it? (Perl in a Nutshell,
2nd Ed, June 2002, O'Reilly.)

Thanks.

 

[xhoster]

My PERL regex, in attempting to identify a preceding backslash in the
js code, I WANT to look like this
s/(?<[^\\])\/\/.*$//x
to identify two slashes on a line but not immediately preceded by a
backslash.

In that case, wouldn't it make more sense to use negative look-behind?
If the line starts with //, then it is not preceded by a backslash, but
it is not preceded by a non-backslash, either.

But the perl compiler tells me it doesn't understand:-
Sequence (?<[...) not recognized in regex; [line cont..]
marked by <-- HERE in m/(?<[ <-- HERE ^\\])//.*$/

Incidentally, if I replace the awkward "not-a-backslash" section of
the regex with a more simple "an x" section, I still get the same
compilation error, i.e.
replace "[^\\]" with "x"

"perl -v" tells me
This is perl, v5.8.8 built for MSWin32-x86-multi-thread
and
Binary build 820 [274739] provided by ActiveState
http://www.ActiveState.com
Built Jan 23 2007 15:57:46

It's not my reference book that's wrong is it? (Perl in a Nutshell,
2nd Ed, June 2002, O'Reilly.)

The perldoc perlre says the positive look behind syntax is (?<=pattern).
If your reference book omits then "=", then it is wrong.

Xho

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[Mumia W.]

Can anyone tell me why my version of Perl won't compile a so-called
positive lookbehind assertion.

Specifically, I want to strip JavaScript comments from a file,
[...]
But the perl compiler tells me it doesn't understand:-
Sequence (?<[...) not recognized in regex; [line cont..]
marked by <-- HERE in m/(?<[ <-- HERE ^\\])//.*$/
[...]

Look closely at "perldoc perlre". (?< ...) is not supported. You can use
one of these:

(?<= ...)
(?<! ...)

 

[Cloink]

Thanks for all your help.

Whilst I generally DO recommend O'Reilly books, not only is their Perl
in a Nutshell book badly written, it now appears it is quite crucially
wrong too.

I will try and remember "perldoc perlre", but I don't use Perl day in
day out, so I am likely to forget.

Thanks again.