I can’t get preg_match to test if the entire string matches the regex

I’m using this regular expression to test if a username is valid:

[A-Za-z0-9 _]{3,12} when I test it for matches in a text editor with the string test'ing, it highlights ‘test’ and ‘ing’, but when I use the following code in PHP:

if(!preg_match('/[A-Za-z0-9 _]{3,12}/', $content) where $content is test'ing and it should return FALSE, it still returns true.

Is there something wrong with my regular expression? I need:

• Minimum length 3, max 12 {3,12}
• No spaces/underscores in front or after the string, and no spaces/underscores in a row anywhere (I’m using additional checks for this because I’m not very good with regex)
• Only alphanumerics, spaces and underscores allowed [A-Za-z0-9 _]

Thanks in advance…

Answer

You’re missing the anchors in the regular expression, so the regex can comfortably match 3 characters in the character class anywhere in the string. This is not what you want. You want to check if your regex matches against the entire string. For that, you need to include the anchors (^ and $).

if(!preg_match('/^[A-Za-z0-9 _]{3,12}$/', $content)
                 ^                   ^

^ asserts the position at the beginning of the string and $ asserts position at the end of the string. It’s important to note that these meta characters do not actually consume characters. They’re zero-width assertions.

Further reading:

• Regex Anchors on regular-expressions.info
• The Stack Overflow Regular Expressions FAQ