Question

Not a big user of RegEx – never really understood them! However, I feel the best way to check input for a username field would be with one that only allows Letters (upper or lower), numbers and the _ character, and must start with a letter as per the site policy. The My RegEx and code is as such:

var theCheck = /[a-zA-Z]|\d|_$/g;
alert(theCheck.test(theUsername));

Despite trying with various combinations, everything is returning “true”.

Can anyone help?

Answer 1

Your regex is saying “does theUsername contain a letter, digit, or end with underscore”.

Try this instead:

var theCheck = /^[a-z]([a-z_\d]*)$/i; // the "i" is "ignore case"

This says “theUsername starts with a letter and only contains letters, digits, or underscores”.

Note: I don’t think you need the “g” here, that means “all matches”. We just want to test the whole string.