You need to be sure that the whole number is either surrounded by , or the beggining or end of line:

$thestring = "15,130,213";
if ($thestring =~ /(^13,)|(,13,)|(,13$)/) { print "13 is in $thestring
+"; }
else { print "13 is not in $thestring"; }
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This is not a very good regexp but it's only an example. I suggest to read more about regular expressions, then start asking question about the difficult parts not the standards covered in almost every regex tutorial.