What does the caret ^ in (?^:…) mean in the string form of a Perl qr// Regex?
perl, regex
Solution
Basically it means the default flags apply (even if it gets interpolated into a regex that specifies differently). Before it was introduced, qr would produce something like `(?-ismx:` and a new flag being added to Perl would make that change, which m ade keeping tests up to date a pain.
http://perldoc.perl.org/perlre.html#Extended-Patterns:
Starting in Perl 5.14, a "^" (caret or circumflex accent) immediately after the "?" is a shorthand equivalent to d-imnsx . Flags (except "d" ) may follow the caret to override it. But a minus sign is not legal with it.
Problem
Consider this script, which is based on an answer to SO 267399 about parsing Roman numbers, though the parsing of Roman numbers is incidental to this question. ``` #!/usr/bin/env perl # # Based on answer to SO 0026-7399 use warnings; use strict; my $qr1 = qr/(?i:M{1,3})/; my $qr2 = qr/(?i:C[MD]|D?C{1,3})/; my $qr3 = qr/(?i:X[CL]|L?X{1,3})/; my $qr4 = qr/(?i:I[XV]|V?I{1,3})/; print "1000s: $qr1\n"; print " 100s: $qr2\n"; print " 10s: $qr3\n"; print " 1s: $qr4\n"; # This $qr is too simple — it matches the empty string #my $qr = qr/($qr1?$qr2?$qr3?$qr4?)/; my $qr = qr/\b((?:$qr1$qr2?$qr3?$qr4?)|(?:$qr2$qr3?$qr4?)|(?:$qr3$qr4?)|(?:$qr4))\b/; print " Full: $qr\n"; while (<>) { chomp; print " Line: [$_]\n"; while ($_ =~ m/$qr/g) { print "Match: [$1] found in [$_] using qr//\n"; } } ``` Given the data file below, the first three lines each contain a Roman number. ``` mix in here no mix in here mmmcmlxxxix minimum ``` When run with (home-built) Perl 5.22.0 on a Mac now running macOS Sierra 10.12.4, I get output like this (but the version of Perl is not critical): ``` 1000s: (?^:(?i:M{1,3})) 100s: (?^:(?i:C[MD]|D?C{1,3})) 10s: (?^:(?i:X[CL]|L?X{1,3})) 1s: (?^:(?i:I[XV]|V?I{1,3})) Full: (?^:\b((?:(?^:(?i:M{1,3}))(?^:(?i:C[MD]|D?C{1,3}))?(?^:(?i:X[CL]|L?X{1,3}))?(?^:(?i:I[XV]|V?I{1,3}))?)|(?:(?^:(?i:C[MD]|D?C{1,3}))(?^:(?i:X[CL]|L?X{1,3}))?(?^:(?i:I[XV]|V?I{1,3}))?)|(?:(?^:(?i:X[CL]|L?X{1,3}))(?^:(?i:I[XV]|V?I{1,3}))?)|(?:(?^:(?i:I[XV]|V?I{1,3}))))\b) Line: [mix in here] Match: [mix] found in [mix in here] using qr// Line: [no mix in here] Match: [mix] found in [no mix in here] using qr// Line: [mmmcmlxxxix] Match: [mmmcmlxxxix] found in [mmmcmlxxxix] using qr// Line: [minimum] ``` The only part of the output that I don't understand is the caret `^` in the `(?^:…)` notation. I've looked at Perl documentation for `perlre` and `perlref` and even the section of `perlop` on 'Regex quote-like operators' without seeing this exemplified or explained. (I also checked the resources suggested by SO when you ask a question about regexes. The `(?^:` string is carefully designed to give search engines conniptions.) There are two parts to my question: - What is the significance of the caret in `(?^:…)` and what caused it to be added to the `qr//` regexes? - If it matters, how do you stop it from being added to the `qr//` regexes?