Regex for a username?

Question

In C#, I am trying to validate a username with the following restrictions:

• ​​Uppercase and lowercase English letters (a-z, A-Z)
• Digits 0 to 9
• Acceptable Characters ! # $ % & ' * + - / = ? ^ _ ` { | } ~ \ , . @ ( ) < > [ ]

The username has no restrictions on what character it can start with, all of the above are acceptable. Can you help me come up with this regex, especially the characters I would have to escape because they already mean something in Regex rules?

Answer 1

In a character class, you just need to look out for ^, -, ] and \.

The first three must be escaped or placed in unambiguous positions:

• for ^ that's anywhere but the first character inside the class,
• for - it's at the start or end of the class,
• for ] it's at the start of the class (this is possible at least in .NET but not in JavaScript, for example).

The backslash must always be escaped.

So this works:

^[]A-Za-z0-9!#$%&'*+/=?^_`{|}~\\,.@()<>[-]*$

but I would use

^[A-Za-z0-9!#$%&'*+/=?^_`{|}~\\,.@()<>[\]-]*$

for portability.

Debuggex Demo