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, as excluding + [+]{1,2} and () [(]{1} [)]{1} doesn't make much sense because I'm trying to make general statements in no particular order so that I would be able to chain it.

Answer 1

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