parsing from the right instead of the left

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

How can parse a string from the right instead of the left? What I'm trying to get is get the extension of a file contained in an URL, such as http://www.test.com/tmp/foo.bar. Here's my kludge: my ($ext) = $url =~ /\.(.+)$/; while ( $ext =~ /\./ ) { ($ext) = $ext =~ /\.(.+)$/; } In Korn Shell, I could use something like ${url%%/./}, but I'm not sure if that's right off the top of my head. Thanks

Edit: 2001-03-03 by neshura

Comment on parsing from the right instead of the left

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RE: parsing from the right instead of the left by chromatic (Archbishop) on Sep 23, 2000 at 03:20 UTC
Writing a reverse regex seems like too much work for a lazy Perl hacker. I think you're on the right track, especially with the end-of-line anchor: `(my $ext = $url) =~ m!\.([^/]+)$!;` With greedy quantifiers like + and *, the regex engine parses from right to left anyway. So take advantage of that. Somehow, that construct isn't behaving for me after a long week, so here's another approach: `#!/usr/bin/perl -w use strict; print "Url: "; chomp(my $url = <STDIN>); if ($url =~ m!\.([^/]+)$!) { print "Ext: $1\n"; } else { print "Not found.\n"; }` [download]	[reply] [d/l] [select]
RE: parsing from the right instead of the left by Adam (Vicar) on Sep 23, 2000 at 00:48 UTC
reverse the string. There was also a post around here, sexeger, about this kind of thing.	[reply]
RE: parsing from the right instead of the left by japhy (Canon) on Sep 23, 2000 at 01:26 UTC
I definitely suggest you check out Sex, Eger!, my paper (soon to be at perl.com!) on reversing regular expressions to match the last of something in a string. If you want to match forward, then you want to match the last sequence of non-. characters in a string. This can be matched by `($ext) = $str =~ /([^.]+)$/`. If you want to reverse it, simply do `$ext = scalar reverse( reverse($str) =~ /([^.]+)/)`. `$_="goto+F.print+chop;\n=yhpaj";F1:eval`	[reply]