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Working of \Q in m//

by sg (Pilgrim)
on Oct 15, 2007 at 07:20 UTC ( #644846=perlquestion: print w/replies, xml ) Need Help??

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

In the following code, what does \Q do to make things work as expected and why do the other attempts fail at working as expected?

UPDATE: figured out the explicit versions

UPDATE: Got the \Q part after reading the link given by Corion: \Q makes $x into $x='E\\:\\\\th\\\\foo'

#!c:/opt/perl/bin/perl use warnings; use strict; my $t='E:\\th\\foo\\bl\\be'; my $x='E:\\th\\foo'; print "t:$t, x:$x\n"; # m// should show that $t starts with $x $t =~ m,^$x, and print "matched\n" or print "didn't match\ +n"; $t =~ m,^\Q$x, and print "matched with Q\n" or print "didn't match +with Q\n"; $t =~ m,^\$x, and print "matched with 1 \\\n" or print "didn't match +with 1 \\\n"; $t =~ m,^\\$x, and print "matched with 2 \\\n" or print "didn't match +with 2 \\\n"; # tried with 3 and 4 \ too # being explicit on right (no quotes) $t =~ m,E:\\th\\foo, and print "yes (explicit)\n" or print "no (explic +it)\n"; $t =~ m,E\:\\th\\foo, and print "yes (1 \\ :)\n" or print "no (1 \\ :) +\n"; $t =~ m,E\\:\\th\\foo, and print "yes (2 \\ :)\n" or print "no (2 \\ : +)\n"; __END__ t:E:\th\foo\bl\be, x:E:\th\foo didn't match matched with Q didn't match with 1 \ didn't match with 2 \ yes (explicit) yes (1 \ :) no (2 \ :)

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Re: Working of \Q in m//
by Corion (Patriarch) on Oct 15, 2007 at 07:37 UTC

    See perlre, the entry for \Q. As the documentation says, it disable[s] pattern metacharacters till \E, which in your case means that the backslash retains its literal meaning instead of being used as a quoting character in a regular expression.

Re: Working of \Q in m//
by ikegami (Patriarch) on Oct 15, 2007 at 19:20 UTC
    $\ = "\n"; my $x='E:\\th\\foo'; print qr/^$x/; # start,"E",":",tab,"h",formfeed,"o","o" print qr/^\Q$x/; # start,"E",":","\","t","h","\","f","o","o" print qr/^\$x/; # start,"$","x" print qr/^\\$x/; # start,"\","E",":",tab,"h",formfeed,"o","o" print qr/E:\\th\\foo/; # "E",":","\","t","h","\","f","o","o" print qr/E\:\\th\\foo/; # "E",":","\","t","h","\","f","o","o" print qr/E\\:\\th\\foo/; # "E","\",":","\","t","h","\","f","o","o"

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