What does \d+ mean in regular expression terms?

\d is a digit (a character in the range 0-9), and + means 1 or more times. So, \d+ is 1 or more digits.

This is about as simple as regular expressions get. You should try reading up on regular expressions a little bit more. Google has a lot of results for regular expression tutorial, for instance. Or you could try using a tool like the free Regex Coach that will let you enter a regular expression and sample text, then indicate what (if anything) matches the regex.

\d is called a character class and will match digits. It is equal to [0-9].

+ matches 1 or more occurrences of the character before.

So \d+ means match 1 or more digits.

\d means 'digit'. + means, '1 or more times'. So \d+ means one or more digit. It will match 12 and 1.