regexp matching with "\[" in the substring to be matched

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

hello, below is a code which logic is unexplicable to me:

#!/bin/perl -w
$p= "horraen=sumsel\[13\]\)\n";
$m="sumsel\[13\]";
print join("","this is \$m:",$m,"\n");
print $p;
if ($p=~m/en=$m\)/) {print "yeah!\n";}
else {print "why isn't it matching?\n";}
$m=~s/\[/\\\[/g;
$m=~s/\]/\\\]/g;
print join("","this is \$m:",$m,"\n");
if ($p=~m/en=$m\)/) {print "yeah!\n";}
else {print "what will I do now?\n";}
[download]

Why do I need the add an addition "\\" before the sqare braces (in the substring) when matching the substring? -ISAI student

Comment on regexp matching with "\[" in the substring to be matched Download Code

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Re: regexp matching with "\[" in the substring to be matched by rev_1318 (Chaplain) on Jul 05, 2005 at 11:54 UTC
Because your var (`$p`) contains a backslash follows by a square bracket (`\[`). `$m` gets interpolated in the RE, so in the first case, the `\[` will be interpolated as only a square bracket, and you lose the backslash. Paul	[reply] [d/l] [select]
Re: regexp matching with "\[" in the substring to be matched by prasadbabu (Prior) on Jul 05, 2005 at 12:13 UTC
As rev_1318 said, you should always escape the special characters by giving backslash. Also you go through perlre. Prasad	[reply]
Re: regexp matching with "\[" in the substring to be matched by nobull (Friar) on Jul 05, 2005 at 16:39 UTC
`$p="horraen=sumsel\[13\]\)\n"; $m="sumsel\[13\]";` [download] The above are exactly equivalent to `$p="horraen=sumsel[13])\n"; $m="sumsel[13]";` [download] If you really want backslashes in the strings you need to double them or use single quotes. `$p="horraen=sumsel\\[13\\]\\)\n"; $m="sumsel\\[13\\]";` [download] ..or.. `$p='horraen=sumsel\[13\]\)' . "\n"; $m='sumsel\[13\]';` [download] Actually since `$m` is a regex it's more ideomatic to use `qr//` unless there's a reason you can't. Also I don't think you really wanted backslashes in `$p`. `$p="horraen=sumsel[13])\n"; $m=qr/sumsel\[13\]/;` [download]	[reply] [d/l] [select]
Re: regexp matching with "\[" in the substring to be matched by tlm (Prior) on Jul 06, 2005 at 02:12 UTC
Also, you can use `\Q...\E` to quote otherwise regex-meaningful characters. E.g. `$p = "horraen=sumsel[13])\n"; # no backslashes required $m = qr/\Qsumsel[13]\E/; # actually in this case qr/\Qsumsel[13]/ is +enough print $p; if ( $p =~ /$m\)/ ) { print "yeah!\n"; } else { print "why isn't it matching?\n"; } __END__ horraen=sumsel[13]) yeah!` [download] See also `quotemeta`. the lowliest monk	[reply] [d/l]