Regular expression that accepts letters, numbers and underline

Question

How do I make the ER below accept letters, numbers and underline, and continue not allowing the other characters?

preg_match('/[^a-z\d]/', $_POST['login'])

Answer 1

The clasps ([]) define a character class, that is, accepts whatever is inside them. Ex: [ab] means "the letter a or the letter b".

But when the first character inside the brackets is a ^, you will be denying what is inside it. Ex: [^ab] means "anything that is not a nor b".

So in doing [^a-z\d], you are rejecting the lower case letters and digits. To accept letters, numbers and underline, you must include them in the brackets and remove the ^, then I would be [A-Za-z0-9_].

I’m assuming you want to accept strings with more than one character, so use the quantifier +, meaning "one or more occurrences". Then the expression is [A-Za-z0-9_]+.

Since these are the only characters allowed, use also ^ out of of the brackets and at the beginning of the expression, because it means beginning of the string, and at the end put $, which means the end of the string. The complete expression is:

preg_match("/^[A-Za-z0-9_]+$/", $_POST['login'])

That is: one or more occurrences (+) letters, numbers or underline ([A-Za-z0-9_]), from the beginning (^) at the end ($) string.