Re (using assertions): pattern matching a limited number of times

I guess this would be a good place for assertions. An assertion is a piece of Perl code embedded in the regex, which says that a match may or may not succeed.

Now the current (perl5) syntax for a regex assertion is very awkward. This will do it:

/(?(?{NOTCOND})(?!))/
[download]

NOTCOND is perl code producing a true when you want the assertion to fail, because only in that case an attempt to match the regex snippet /(?!)/ is made, and this is a lookahead that never matches.

So, you can try this:

my $i = 0;
s/ab(?(?{++$i>5})(?!))/cd/g;
[download]

This will indeed stop matching after the fifth match, producing the desired result; but it will not stop trying: it will go through the whole string, and if your string contains 50 substrings "ab", then the assertion code will be called 50 times. So for long strings, it won't be too efficient.

Comment on Re (using assertions): pattern matching a limited number of times Select or Download Code

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Re2: pattern matching a limited number of times by blakem (Monsignor) on Sep 14, 2002 at 09:42 UTC
How about a modified version of japhy's code. `#!/usr/bin/perl -wT use strict; my $string = 'abc ' x 10; my $search = 'ab'; my $replace = 'CD'; print "$string\n"; my $i = 0; while ($string =~ /$search/gi) { substr($string, $-[0], $+[0]-$-[0], $replace); last if ++$i == 4; pos($string) = $-[0] + 1; } print "$string\n"; __END__ abc abc abc abc abc abc abc abc abc abc CDc CDc CDc CDc abc abc abc abc abc abc` [download] Update: Doh! I didn't see your very similar solution before I posted.... -Blake	[reply] [d/l]
Re: Re2: pattern matching a limited number of times by bart (Canon) on Sep 14, 2002 at 10:14 UTC
Doh! I didn't see your very similar solution before I posted.... It is very similar, isn't it? And I hadn't seen yaphy's solution before I posted mine. Now, it left me wondering: is setting pos() necessary, or will Perl DWIM, and continue matching at the same point in the (original) string without help? That's what s///g does. Well, it turns out that it appears to be using an offset in the string to keep track of where it was. So it is necessary. `$_ = 'abxy' x 12; for(my $i = 0; m/ab/g and $i++<5;) { substr($_, $-[0], $+[0]-$-[0]) = 'abXab'; } print;` [download] Result: `abXabXabXabXabXabxyabxyabxyabxyabxyabxyabxyabxyabxyabxyabxyabxy` Not good. `$_ = 'abxy' x 12; for(my $i = 0; m/ab/g and $i++<5;) { substr($_, $-[0], $+[0]-$-[0]) = 'abXab'; pos = $-[0]+5; } print;` [download] Result: `abXabxyabXabxyabXabxyabXabxyabXabxyabxyabxyabxyabxyabxyabxyabxy` Good. p.s. I noticed this, which is also very awkward: `my $length = length(substr($_, 2, 2) = 'abXab'); print $length;` [download] prints 2, not 5. What happened to the rule: the value of an assignment as an expression, is what you assign?	[reply] [d/l] [select]
Re: Re: Re2: pattern matching a limited number of times by BrowserUk (Patriarch) on Sep 14, 2002 at 10:48 UTC
p.s. I noticed this, which is also very awkward: Aren't you simply being bitten by the same bug I reported here in thread which you commented on here? For reference, Perlbug assigned a ticket ID of [perl #16834]. Well It's better than the Abottoire, but Yorkshire!	[reply]


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