What is the difference between character classes \d and [:digit:] in a regular expression?

Question

I studied a tutorial for vim syntax files and found this example:

" Floating point like number with E and no decimal point (+,-)
syn match celNumber '[-+]\=\d[[:digit:]]*[eE][\-+]\=\d\+'

I was surprised that it uses both \d and [:digit:]. I thought they mean the same thing. The vim help for pattern says:

Character classes:
      magic   nomagic   matches ~
...
\d    \d      \d        digit:    [0-9]

And for [:...:] character classes:

- A character class expression is evaluated to the set of characters
  belonging to that character class.  The following character classes
  are supported:
Name          Func         Contents ~
...
[:digit:]     [:digit:]    decimal digits '0' to '9'

So what is the difference? Can't the example pattern \d[[:digit:]]* be simplified to \d\+?

Answer 1

There is no difference in which symbols they encompass; both denote the digits from 0 to 9. They only differ in where they can resp. must be used. \d can not be used inside a collection, i.e., /[\d]/ will search for backslashes and lower case Ds, but not for digits. In contrast to this, [:digit:] must be used inside a collection, i.e., only /[[:digit:]]/ will search for a digit -- /[:digit:]/ would search for one of the letters d, g, i, t or for a colon.

And yes, \d[[:digit:]]* can be simplified to \d\+.