Regular expressions mystery

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

Hi Monks, I have a string that can have the form:

$str = 'LLLLLLLLLLLLLLLLLLLOOOOOOOOOOOOO';
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$str = 'LLLLLLLLLLCOOOOOOOOOOOOOOOOOOOOOO';
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I wrote this reg exp, but it does not seem to work:

if($str=~/^(L+)/ or $str=~/^(L+C)/)
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Can you tell me why? I would like it that it can catch both possibilities, either being LO or LCO and I thought this or was what I needed, but it does not work and it fails for both.
Thanks

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Re: Regular expressions mystery by soonix (Chancellor) on Jul 01, 2018 at 13:31 UTC
you might want to match /^(L+C?)/ . Another way: if it isn't possible to combine it into a single regex, change the order of the conditions such that the longer comes earlier.	[reply]
Re: Regular expressions mystery by Lotus1 (Vicar) on Jul 01, 2018 at 12:53 UTC
It 'worked' for me. You might want to let us know what you are trying to capture or accomplish and show a complete program. `use warnings; use strict; foreach my $str ( 'LLLLLLLLLLLLLLLLLLLOOOOOOOOOOOOO', 'LLLLLLLLLLCOOOO +OOOOOOOOOOOOOOOOOO' ){ if($str=~/^(L+)/ or $str=~/^(L+C)/) { print "matched '$1'\n"; } } __END__ matched 'LLLLLLLLLLLLLLLLLLL' matched 'LLLLLLLLLL'` [download]	[reply] [d/l]
Re^2: Regular expressions mystery by AnomalousMonk (Archbishop) on Jul 01, 2018 at 13:28 UTC
`AnonyMonk`: Note that you will never capture the `C` character in the string (if this is what you want to do) because the `$str =~ /^(L+)/` expression will always match first in `if ($str =~ /^(L+)/ or $str =~ /^(L+C)/) { ... }` for the strings you give as examples. If you want the `C` captured, reverse the order of the matches `if ($str =~ /^(L+C)/ or $str =~ /^(L+)/) { ... }` or much better yet IMHO, just use a single regex with an optional `C` match `if ($str =~ /^(L+C?)/) { ... }` (See the `?` quantifier in perlre.) Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^3: Regular expressions mystery by Anonymous Monk on Jul 01, 2018 at 17:01 UTC
Thank you very much! Good point	[reply]
Re: Regular expressions mystery by Anonymous Monk on Jul 01, 2018 at 11:17 UTC
Aha, it should have been `\|` instead of `or`. I did not know that.	[reply] [d/l] [select]
Re^2: Regular expressions mystery by haj (Vicar) on Jul 01, 2018 at 18:33 UTC
Aha, it should have been \| instead of or. I did not know that. What makes you say that? `\|` is a "bitwise or" which isn't of any benefit here. `or` is quite fine!	[reply]