This function matches strings with underscores, because the \w escape sequence allows underscores:

A "word" character is any letter or digit or the underscore character, that is, any character which can be part of a Perl "word".

which are against this condition in the kata's description:

• No whitespaces/underscore

For example, this regex expression matches the string "string_with_underscores".

Edit: \d is unneeded in the PCRE expression. The \w escape character already covers digits.