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Escaping a special variable embedded in a regex

rpaskudniak
User

Oct 19, 2017, 3:02 PM

Post #1 of 4 (5377 views)

Escaping a special variable embedded in a regex	Can't Post

Although I have posted questions in the intermediate and advanced forums and I feel really stupid about this but this question really does belong in beginners. But here it is.

In preparing for a Perl test I encountered a pretty good exercise on this Wiley site http://www.wiley.com/legacy/compbooks/chapman/perl/exercises2.html. It involves matching a bunch of strings against a bunch of regular expressions.

One of these patterns is:

Code

/[^\w+$]/

Chasing a strange looking error, I put the whole list into an array of regexes looking like:

Code

 ... 
 11     qr/^\w+$/, 
 12     qr/[^\w+$]/,     <=== Note: In vim, the $] show up in white; can't mark that up here 
 13     qr/\d/, 
 ...

When I run that do-nothing code I get this compile error:

Code

$ ./just_patterns.pl 
Unmatched [ in regex; marked by <-- HERE in m/[ <-- HERE ^\w+5.022004/ at ./just_patterns.pl line 7.

I've attached a screen shot of the vim session. Notice the color change on line 12.

That 5.022004 gives me a broad hint: It happens to be the version of Perl I'm using. So the pattern contains an embedded special variable $] .

NOW I come to the question: How would I escape that special variable $] inside the regex pattern?

(If you thing this is verbose, be glad I realized the cause of the error before posting. Angelic

)

Much thanks for an idea here!
--------------------
-- Rasputin Paskudniak (In perpetual pursuit of undomesticated, semi-aquatic avians)

Attachments:

just-patterns.png (14.6 KB)

Laurent_R
Veteran / Moderator

Oct 19, 2017, 11:23 PM

Post #2 of 4 (5368 views)

Re: [rpaskudniak] Escaping a special variable embedded in a regex [In reply to]	Can't Post

What do you want to match exactly with the:

Code

qr/[^\w+$]/

pattern?

There is most probably something wrong with it, but trying to fix it depends on the intended goal, which isn't clear to me.

If you want to match a literal [, then you need to escape it and it does not make sense to have a beginning of stgring anchor after it.

Square brackets introduce presumably a character class, but why would you want ^ or $ (beginning and end of string anchors) in a character class? It doesn't make much sense.

If you're looking for literal ^ and $ (sound unlikely, but who knows?), then I guess you would need to escape them (I can't test right now in public transportation toward my $work).

So, I would say, tell us what it is supposed to match, then we can think about how to fix it.

BillKSmith
Veteran

Oct 20, 2017, 4:11 AM

Post #3 of 4 (5365 views)

Re: [rpaskudniak] Escaping a special variable embedded in a regex [In reply to]	Can't Post

I assume that you wish to match any character is which is neither a word-character nor a plus sign nor a dollar sign.

Here is one way to do that.

Code

C:\Users\Bill\forums\guru>type rpaskudniak.t 
use strict; 
use warnings; 
use Test::More tests=>4; 
my $class =     q(^A-Za-z0-9_+$); 
my $regex = qr/[$class]/; 
my $string; 
 
$string = q(b); 
ok( $string !~ $regex, $_ ); 
 
$string = q(+); 
ok( $string !~ $regex, $_ ); 
 
$string = q($); 
ok( $string !~ $regex, $_ ); 
 
$string = q(This should match!); 
ok( $string =~ $regex, $_ ); 
 
 
C:\Users\Bill\forums\guru>perl rpaskudniak.t 
1..4 
ok 1 
ok 2 
ok 3 
ok 4

Note: Backslash does not work as an escape inside a character class.

UPDATED: Removed unnecessary slashes from matches.
Good Luck,
Bill

(This post was edited by BillKSmith on Oct 20, 2017, 4:49 AM)

BillKSmith
Veteran

Oct 23, 2017, 6:10 AM

Post #4 of 4 (5238 views)

Re: [rpaskudniak] Escaping a special variable embedded in a regex [In reply to]	Can't Post

I just reread the site you linked to. It contains a link to a 'Hint/Solution'. That contains a link to a script which demonstrates how the authors intended the REGEXs to be used. I think it is a bad idea to treat a REGEX as a single quoted string, but it does avoid your problem.

Code

check_one('[^\w+$]'); 
 
... 
 
sub check_one { 
    my $re = shift; 
    print "$ARG =~ $re"; 
   ... 
}

Good Luck,
Bill

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