$p="horraen=sumsel\[13\]\)\n";
 $m="sumsel\[13\]";

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The above are exactly equivalent to 

 $p="horraen=sumsel[13])\n";
 $m="sumsel[13]";

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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\\]";

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..or.. 
 $p='horraen=sumsel\[13\]\)' . "\n";
 $m='sumsel\[13\]';

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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\[13\]/;

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In reply to Re: regexp matching with "\[" in the substring to be matched by nobull
in thread regexp matching with "\[" in the substring to be matched by ISAI student

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