What is double plus in regular expressions?

Question

I have been seeing this in some PHP script:

[a-zA-Z0-9_]++

What does the double plus mean?

Answer 1

To give you a very simple example:

Let's say you have a string "123". The matched characters have a ^ underneath in the following examples.

1. Regex: \d+?. partial match!

   123  # The \d+? eats only 1 because he's lazy (on a diet) and leaves the 2 to the .(dot).
   ^^   # This means \d+? eats as little as possible.
   

2. Regex: \d+. full match!

   123  # The \d+ eats 12 and leaves the 3 to the .(dot).
   ^^^  # This means \d+ is greedy but can still share some of his potential food to his neighbour friends.
   

3. Regex: \d++. no match!

   123  # The \d++ eats 123. He would even eat more if there were more numbers following. 
        # This means \d++ is possessive. There is nothing left over for the .(dot), so the pattern can't be matched.