Mike Griffin

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Commenting Regexes

• programming

Regexes are really useful but as the old saying goes:

Some people, when confronted with a problem, think “I know, I’ll use regular expressions.” Now they have two problems.

One of the issues that I have with them is that sometimes they can take a bit of testing to get them to work. When you come back to them after a while, it’s like looking at hieroglyphics, with little chance of understanding them again.

To try and combat this, I’ve been commenting my regexes over multiple lines, like this:

$findGRpattern = "/\b".          # start the search on a word boundary
                 "(?<![.])".     # a negative lookbehind to exclude numbers with decimal places
                 "(".            # start the capture
                 "[A-HJ-Z]?".    # an optional capital letter that is not an I
                 "\d{3,5}".      # 3, 4, or 5 numbers
                 "\s?".          # an optional space
                 "\d{3,5}".      # another 3, 4, or 5 numbers
                 ")".            # stop the capture
                 "[A-Z]?".       # an optional capital letter
                 "\b/";          # finish the search at a word boundary

I’ve broken each section of the regex up onto its own line and added a short comment about what it is supposed to do. I’ve found that this helps when I come back and have to debug or add something into the regex at a later stage.

How do you go about making your regexes future friendly?