REGEXP: only need last matching string

jeanluca has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: REGEXP: only need last matching string by inman (Curate) on Dec 23, 2005 at 12:05 UTC
$1 will contain the last match if you execute the regexp in list context. `(my $dum) = $str =~/abc\s(\d+)/g ; print "dum is $1\n" ;` [download] gives `dum is 14` [download]	[reply] [d/l] [select]
Re^2: REGEXP: only need last matching string by blazar (Canon) on Dec 23, 2005 at 12:24 UTC
Then `() = $str =~ /abc\s(\d+)/gs;` [download] will suffice, i.e. no need for the `$dum` variable.	[reply] [d/l] [select]
Re: REGEXP: only need last matching string by prasadbabu (Prior) on Dec 23, 2005 at 12:16 UTC
TIMTOWTDI `#! /usr/bin/perl $str = <<HERE ; abc 10 abc 11 abc 12 abc 13 abc 14 HERE ($dum) = $str =~/abc\s(\d+)$/; print "dum is $dum\n" ;` [download] Prasad	[reply] [d/l]
Re: REGEXP: only need last matching string by salva (Canon) on Dec 23, 2005 at 12:05 UTC
try this: `($dum) = $str =~ /^.*abc\s(\d+)/s` [download] though, I am not sure about the eficience of this regular expresion.	[reply] [d/l]
Re^2: REGEXP: only need last matching string by duff (Parson) on Dec 23, 2005 at 20:07 UTC
When I read this, I thought "Don't be ridiculous! Yours is probably the most efficient method suggested!" Then I set about benchmarking the various suggestions to prove it. Alas, yours is only the second most efficient (in terms of time): Rate inman blazar2 blazar1 jeanluca thedoe2 prasadbabu drmoron salva thedoe1 inman 107580/s -- -7% -28% -33% -51% -54% -72% -77% -78% blazar2 115313/s 7% -- -23% -28% -48% -51% -70% -75% -76% blazar1 149209/s 39% 29% -- -7% -32% -37% -61% -68% -69% jeanluca 160157/s 49% 39% 7% -- -27% -32% -58% -65% -67% thedoe2 220552/s 105% 91% 48% 38% -- -6% -43% -52% -54% prasadbabu 235856/s 119% 105% 58% 47% 7% -- -39% -49% -51% drmoron 384090/s 257% 233% 157% 140% 74% 63% -- -16% -20% salva 459627/s 327% 299% 208% 187% 108% 95% 20% -- -5% thedoe1 481345/s 347% 317% 223% 201% 118% 104% 25% 5% -- Read more... (3 kB) duff	[reply] [d/l] [select]
Re^3: REGEXP: only need last matching string by japhy (Canon) on Dec 24, 2005 at 21:23 UTC
I can see NO reason why thedoe1 should be performing any better than salva's code. `salva => sub { (my $dum) = $str =~ /^.abc\s(\d+)/s; }, thedoe1 => sub { (my $dum) = $str =~ /(?<!abc).abc\s(\d+)/gs; },` [download] Those regexes are equivalent. In fact, I can't understand why in the world thedoe used a look-behind there. It accomplishes nothing, since the first place the regex tries to match is at the beginning of the string. The only difference that it could make is if `pos($str)` is something other than 0, and then that means it would not necessary operate properly (insofaras what was requested from the regex). Sorry to sound grumpy, but this is a misuse of a look-behind (and the /g modifier) that I think should be pointed out. There's no voodoo going on. Jeff `japhy` Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and `perl` hacker How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart	[reply] [d/l] [select]
Re^3: REGEXP: only need last matching string by salva (Canon) on Dec 25, 2005 at 10:35 UTC
well, you are benchmarking a corner case, that's when the last `abc \d+` is at the end of the string. When I said that I didn't know about its performance I was thinking on some not so convenient case, for instance, if the matching substr is near the beginning, in that case perl regexp engine is going to backtrack a lot and maybe looping with the OP regexp and discarding all but the last match could perform better.	[reply] [d/l]
Re: REGEXP: only need last matching string by blazar (Canon) on Dec 23, 2005 at 12:22 UTC
`(my $dum) = reverse $str =~/abc\s(\d+)/gs; print "dum is $dum\n"; 1 while $str =~/abc\s(\d+)/gs; print "dum is $1\n";` [download]	[reply] [d/l]
Re: REGEXP: only need last matching string by doctor_moron (Scribe) on Dec 23, 2005 at 12:31 UTC
`#! /usr/bin/perl $str = <<HERE ; abc 10 abc 11 abc 12 abc 13 abc 14 HERE ($dum) = $str =~/\d+$/gs ; print "dum is $dum\n" ;` [download] does it work ? zak	[reply] [d/l]
Re: REGEXP: only need last matching string by thedoe (Monk) on Dec 23, 2005 at 14:26 UTC
You could use a negative look behind assertion to do this: `($dum) = $str =~/(?<!abc).abc\s(\d+)/gs;` To read more about this feature, along with lots more on regex, check out perlre. Update: Another way to do this is explained in Roy Johnson's post on look-ahead and look-behind. I knew I had seen this before, but couldn't find it when I was first answering your question. `/abc(?!.abc)\s(\d+)/s` Not sure which is easiest for you to understand, but either way I would recommend reading that post. (Thanks to prasadbabu for linking to it in a later node so I could find it again!)	[reply] [d/l] [select]