Question

How can I extend already present Regex’s with an attribute telling that the regex can’t exceed a maximum length of (let’s say) 255?

I’ve got the following regex:

([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)

I’ve tried it like that, but failed:

{.,255([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)}

Answer 1

Best way of doing this, if it has to be a solely regex based solution, would be to use lookarounds.

View this example: http://regex101.com/r/yM3vL0

What I am doing here is only matching strings that are at most three characters long. Granted, for my example, this is not the best way to do it. But ignore that, I’m just trying to show an example that will work for you.

You also have to anchor your pattern, otherwise the engine will just ignore the lookaround (do I have to explain this in depth?)

In other words, you can use the following in your regular expression to limit it to at most 255 characters:

^(?!^.{256})([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)

I also feel it is my duty to tell you your regular expression is bad and you should feel bad.