Question

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I am trying to validate a username with the following restrictions:

• Must start with a letter or number
• Must be 3 to 15 characters in length
• Symbols include: . – _ ( ) [ ]
• Symbols cannot be adjacent, but letters and numbers can

Edit:

• Letters and numbers are a-z A-Z 0-9

Been stumped for a while. I’m new to regex.

Answer 1

It is not so clean to express a set of unrelated rules in a single regular expression, but it can be done by using lookaround assertions (Rubular):

@"^(?=[A-Za-z0-9])(?!.*[._()\[\]-]{2})[A-Za-z0-9._()\[\]-]{3,15}$"

Explanation:

(?=[A-Za-z0-9])            Must start with a letter or number
(?!.*[._()\[\]-]{2})       Cannot contain two consecutive symbols
[A-Za-z0-9._()\[\]-]{3,15} Must consist of between 3 to 15 allowed characters

You might want to consider if this would be easier to read and more maintable as a list of simpler regular expressions, all of which must validate successfully, or else write it in ordinary C# code.