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.

This does not seem to account for the case in which an empty string is passed in.

ABOUT RANDOM TEST FAILED!!!!

Finally got it. You should remember, that random test can give you such sequences as:

1. From - to +, e.g. -1, 1, 3, 7 (5 miss)
2. From + to -, e.g. 1, -1, -3, -7 (-5 miss)
3. From zero to smth, e.g. 0, 3, 6, 12 (9 miss)
4. UNCHANGING SEQUENCE, e.g. 2, 2, 2, 2 (2 miss)

I think, most of us got stuck at p.4