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Regular expression for a string with a max length that contains a required part?

Answer 1

+1 A:

The regular expression I'd use would be /^([^\$]*)\$rolename\$([^\$]*)$/, validating the total length of the string externally.

Joaquim Rendeiro 2009-12-03 10:36:33

Sounds right. I'm not sure you need to escape dollars in character groups - `[^$]` might do. I'll add that you want to test the length first, before the regex. It is much cheaper.

Kobi 2009-12-03 10:51:16

In my case it is impossible to validate the string length externally. I'd like to validate an input field in a user interface. The input field and its validation code (regex validation) are generated. I can not change the generation of this code.

Ronald Wildenberg 2009-12-03 11:05:49

@rwwilden - I've added an option for you, but it's ugly. It's odd that you're allowed to control a regex validation, but not maximum length. This is very common for text boxes...

Kobi 2009-12-03 11:46:12

Answer 2

A:

why regex? think simple. depending on your language, check for length using your language's string length methods eg using Python

if len(mystring) <= 20:
     if "$rolename" in mystring:
         print "ok"

your language may have similar methods like index() to find where a substring is in a string.

2009-12-03 10:50:16

I really need a regex. If the problem was this simple to solve, I wouldn't have asked :) See also my comment for the answer by Joachim Rendeiro.

Ronald Wildenberg 2009-12-03 11:07:39

then you should have stated this when you post our question.

2009-12-03 11:08:33

Answer 3

+4 A:

Since you have to use a regex, as you've explained, here's an option:

^(?!.{21,})(.*?)\$rolename\$(.*?)$

This is similar to Joachim's answer, but with a negative lookahead at the beginning. That is: before the regex is matched, we check the string does not have 21 or more characters.

Kobi 2009-12-03 11:35:15

That's exactly what I want, thanks. I was just wondering why you have (.*?) instead of (.*) around $rolename$.

Ronald Wildenberg 2009-12-03 12:17:00

This is a non-greedy capture. I hadn't tested performances, but it should be a little better. It's supposed to stop when it finds `$rolename$`, instead of keep going until the end, failing and back-tracking.

Kobi 2009-12-03 12:20:07

Come to think of it, `(?!.{21})` would work just the same, and also `(?=.{1,20}$)` if you're an optimist :)

Kobi 2009-12-03 12:25:50

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Regular expression for a string with a max length that contains a required part?

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