C# Regex for a username with a few restrictions

Question

Similar to this topic.

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

As an optimization to Mark's answer:

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

Explanation:

(?=.{3,15}$)                   Must be 3-15 characters in the string
([A-Za-z0-9][._()\[\]-]?)*   The string is a sequence of alphanumerics,
                               each of which may be followed by a symbol

This one permits Unicode alphanumerics:

^(?=.{3,15}$)((\p{L}|\p{N})[._()\[\]-]?)*$

This one is the Unicode variant, plus uses non-capturing groups:

^(?=.{3,15}$)(?:(?:\p{L}|\p{N})[._()\[\]-]?)*$