Multiple Conditions in Validation Regex

Question

I am more or less a regex novice. I tried to get the following conditions into a regex:

• No White Space at the beginning and at the end of a string
• Allow all digits
• Allow spaces, points, brackets, plus and minus

Resulted in the following regex:

^\S[\d\/. ()\-+]*\S$

But now i try to apply two more conditions:

• Allow only one or two +
• Allow only one ( and one )

My problem is how to merge those two conditions into the existing regex string above cuz excluding + [+]{1,2} and () [(]{1} [)]{1} doesn't make much sense cuz i try to make general statements in no particular order so that i would be able to chain it. Thanks Ralf

Solution

Use this:

^(?=\S)(?=(?:[^+]*\+){0,2}[^+]*$)(?=(?:[^(]*\()?[^(]*$)(?=(?:[^)]*\))?[^)]*$)[- .()+0-9]*[-.+()0-9]$

In the regex demo you can play with the input to check what matches and doesn't.

Explanation

• The ^ anchor asserts that we are at the beginning of the string
• The lookahead (?=\S) asserts that what follows is a non-space character
• Lookahead (?=(?:[^+]*\+){0,2}[^+]*$) : two + chars at the most
• Lookahead (?=(?:[^(]*\()?[^(]*$) : One ( at the most
• Lookahead (?=(?:[^)]*\))?[^)]*$) : One ) at the most
• [- .()+0-9]* zero or more of the allowed chars
• [-.+()0-9] end with one of the allowed chars that is not a space
• The $ anchor asserts that we are at the end of the string

Reference

• Lookahead and Lookbehind Zero-Length Assertions
• Mastering Lookahead and Lookbehind

Comments

[zx81]

Thank you, glad it helped. :)

[rkoller]

Thanks a lot! Works exactly as planned. But i think i have to work through your explanations to actually understand everything. A few new bits i haven't seen yet. ;)

[zx81]

I recommend you start with the second article in the reference section as the explanation is at a much more comfortable pace. The technique is the same—this is similar to a password-validation regex.

[rkoller]

Yep the first link i already knew, ran across it a few hours ago while researching. But it's a bit too high paced. The other link seems much more suitable for my current level. Perfect. Thanks a lot...