my ($number)= $searchstring=~/\|M\|\(avp-code = 268\)\(flags = 64\)\] 
+= (\d+)/;
#or
my ($number)= $searchstring=~/\Q|M|(avp-code = 268)(flags = 64)]\E = (
+\d+)/;
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The only difficult part is escaping all the characters that have a special meaning in regexes. Easier to use \Q\E to do that automatically