Regex question - identify which pattern comes first

harangzsolt33 has asked for the wisdom of the Perl Monks concerning the following question:

I feel like I have learned a lot in the past 10 years since I started learning Perl, but I still probably don't know more than half of what's possible using regex. I have often come across situations where I needed to identify which pattern occurs first in a string. So, I am not trying to capture a part of the pattern nor am I trying to identify if it occurs at all or where. I am just trying to figure our which of the possible patterns is FIRST in the string. For example:

Sample string: "AB ABDA DCACCB AAA BSAA CAAB ACS ABA DBA BA DASSABACA A"

I'm looking for either: BA[ABC]{2} OR CA[CD]{2} OR DA[SC]{2}

So, I would write: /BA[ABC]{2}|CA[CD]{2}|DA[SC]{2}/

Is there a way to get a return value of 1, 2, or 3 depending on which pattern was matched first? How would I do that?

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Re: Regex question - identify which pattern comes first by ikegami (Patriarch) on May 02, 2026 at 01:53 UTC
`if ( /(BA[ABC]{2})\|(CA[CD]{2})\|(DA[SC]{2})/ ) { my $first = ( defined( $1 ) ? 1 : defined( $2 ) ? 2 : 3 ); ... }` [download] `local our $first; / (?: BA[ABC]{2} (?{ $first = 1; }) \| CA[CD]{2} (?{ $first = 2; }) \| DA[SC]{2} (?{ $first = 3; }) /x;` [download] Update: I had mixed up `(??{ })` and `(?{ })`. Fixed.	[reply] [d/l] [select]
Re^2: Regex question - identify which pattern comes first by harangzsolt33 (Deacon) on May 02, 2026 at 02:15 UTC
Oh wow, that is an amazingly simple solution! Thank you so much!!! :-)	[reply]
Re^3: Regex question - identify which pattern comes first by LanX (Saint) on May 02, 2026 at 13:28 UTC
> that is an amazingly simple solution! Did you try it? These `(??{ $first = 1; })` won't work because they'll include 1,2 or 3 into the regex. Try `(?{ $first = 1; })`; with a single `?` instead. Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery} Thus	[reply] [d/l] [select]
Re^4: Regex question - identify which pattern comes first by harangzsolt33 (Deacon) on May 02, 2026 at 14:20 UTC
Re^5: Regex question - identify which pattern comes first by LanX (Saint) on May 02, 2026 at 14:43 UTC
Some notes below your chosen depth have not been shown here
Re: Regex question - identify which pattern comes first (named captures) by LanX (Saint) on May 02, 2026 at 15:18 UTC
requires 5.10 for named capture groups: This code will even give you the sequence. `use v5.10; use strict; use warnings; my $str = "AB ABDA DCACCB AAA BSAA CAAB ACS ABA DBA BA DASSABACA A"; while ( $str =~ / (?<_1> BA[ABC]{2} ) \| (?<_2> CA[CD]{2} ) \| (?<_3> DA +[SC]{2} ) /gx) { print keys %+, " => ", values %+, " at pos ", pos($str), " \n"; }` [download] `_2 => CACC at pos 13 _3 => DASS at pos 48 _1 => BACA at pos 53` [download] Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery} edit inserted /x for readability	[reply] [d/l] [select]
Re^2: Regex question - identify which pattern comes first (named captures) by ikegami (Patriarch) on May 03, 2026 at 15:53 UTC
Adjusted for OP: `if ( $str =~ / (?: (?<_1> BA[ABC]{2} ) \| (?<_2> CA[CD]{2} ) \| (?<_3> DA[SC]{2} ) ) /x ) { my $first = substr( ( keys( %+ ) )[0], 1 ); ... }` [download] Update: Added missing `)`, as LanX pointed out.	[reply] [d/l] [select]
Re^3: Regex question - identify which pattern comes first (named captures) by LanX (Saint) on May 03, 2026 at 21:29 UTC
You might want to correct this > `(?:` Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery}	[reply] [d/l]
Re: Regex question - identify which pattern comes first by tybalt89 (Monsignor) on May 05, 2026 at 17:20 UTC
`#!/usr/bin/perl use strict; # https://perlmonks.org/?node_id=11167676 use warnings; local $_ = 'AB ABDA DCACCB AAA BSAA CAAB ACS ABA DBA BA DASSABACA A'; my $first = /(BA[ABC]{2})\|(CA[CD]{2})\|DA[SC]{2}/ ? $1 ? 1 : $2 ? 2 : 3 + : 0; print "first is $first\n";` [download] Outputs: `first is 2` [download]	[reply] [d/l] [select]

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