This is a match:
preg_match('/[a-bA-B0-9]+/', 'username')
But this is not:
preg_match('/^[a-bA-B0-9]+$/', 'username')
Why is that?
Are you testing the actual literal 'username'?
/[a-bA-B0-9]+/ will test the existence of a,b,A,B,0,1,2,3,4,5,6,7,8,9 anywhere in the string. So it will match abBa854Abba32, and it will match sjfsgfafnvesv.
/^[a-bA-B0-9]+$/ will test that the enitre string is made of a,b,A,B,0,1,2,3,4,5,6,7,8,9. So it will match abBa854Abba32, and it will not match sjfsgfafnvesv.
Perhaps you meant /^[a-zA-Z0-9]+$/.
The second does not match because you have told the regex to match a sequence that starts with one of those items in the character class. The first letter of your string is "u", and there is no "u" in your character class [a-bA-B0-9].