Question

I don’t understand or see the need for regular expressions.

Can some explain them in simple terms and provide some basic examples where they could be useful, or even critical.

Answer 1

Use them where you need to use/manipulate patterns. For instance, suppose you need to recognise the following pattern:

• Any letter, A-Z, either upper or lower case, 5 or 6 times
• 3 digits
• a single letter a-z (definitely lower case)

(Things like this crop up for zip code, credit card, social security number validation etc.)

That’s not really hard to write in code – but it becomes harder as the pattern becomes more complicated. With a regular expression, you describe the pattern (rather than the code to validate it) and let the regex engine do the work for you.

The pattern here would be something like

[A-Za-z]{5,6}[0-9]{3}[a-z] 

(There are other ways of expressing it too.) Grouping constructs make it easy to match a whole pattern and grab (or replace) different bits of it, too.

A few downsides though:

• Regexes can become complicated and hard to read quite quickly. Document thoroughly!
• There are variations in behaviour between different regex engines
• The complexity can be hard to judge if you’re not an expert (which I’m certainly not!); there are ‘gotchas’ which can make the patterns really slow against particular input, and these gotchas aren’t obvious at all
• Some people overuse regular expressions massively (and some underuse them, of course). The worst example I’ve seen was where someone asked (on a C# group) how to check whether a string was length 3 – this is clearly a job for using String.Length, but someone seriously suggested matching a regex. Madness. (They also got the regex wrong, which kinda proves the point.)
• Regexes use backslashes to escape various things (e.g. use . to mean ‘a dot’ rather than just ‘any character’. In many languages the backslash itself needs escaping.