a question about RE

hello everyone£¡

I have a question about a regular-expression line in "scan.l" file.
It is:
ESCSEQ (\\([^\n]|[0-7]+|x[[:xdigit:]]+))

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

Thanks & Regards!

herdsman sun
Netpower Inc. in Beijing