Regular Expression for alphanumeric and underscores

Question

I would like to have a regular expression that checks if a string contains only upper and lowercase letters, numbers, and underscores.

Answer 1

Um...question: Does it need to have at least one character or no? Can it be an empty string?

^[A-Za-z0-9_]+$

Will do at least one upper or lower case alphanumeric or underscore. If it can be zero length, then just substitute the + for *

^[A-Za-z0-9_]*$

Edit:

If diacritics need to be included (such as cedilla - ç) then you would need to use the word character which does the same as the above, but includes the diacritic characters:

^\w+$

Or

^\w*$