a question about RE

ESCSEQ (\\([^\n]|[0-7]+|x[[:xdigit:]]+))

I can't understand what "[[:xdigit:]]" means?
Would you give me an explanation?

[[:xdigit:]] is one of the magic charaacter classes that POSIX standardized.
It expands to any hex digit, i.e., [0-9a-fA-F]. More precisely, it expands
to any character matched by <ctype.h>'s isxdigit(). The full set of such
expansions are:

  [:alnum:] [:alpha:] [:blank:] [:cntrl:] [:digit:] [:graph:]
  [:lower:] [:print:] [:punct:] [:space:] [:upper:] [:xdigit:]

- Vern