What does the _ in [a-zA-Z0-9_] mean?

Question

What does the underscore mean in the following regex?

[a-zA-Z0-9_]

The _ seems to make no difference so I don\'t understand the purpose

Answer 1

Regular expressions are documented in perlre. That's the place to check whenever you have a question about regular expressions. The Regular-Expressions.info site is very helpful too.

To get you started, the thing you are looking at is called a "character class". Any of the characters inside a character class can match.

You can make a range of characters with the -, so a-z is any of the lowercase letters in that range. A-Z are the uppercase letters and 0-9 are the digits. The _ is a literal underscore. Taken together those are the legal characters for a Perl identifier (variable names and so on). That's the \w character class in the ASCII sense (and not the expanded Unicode sense).

People often use that to match a Perl variable name but there's a rule that people forget. The first character of a user-defined name has to be a letter or underscore (not a digit). That means that you should use a different character class for the initial letter:

 [A-Za-z_][A-Za-z0-9_]*