flex (lexical analyzer) unchanging expressions - reusing definitions

i have operative definition:

identifier   [a-za-z][a-za-z0-9]*

i don't wish keep repeating [a-za-z] [0-9], i finished twin new definitions

digit    [0-9]
valid    [a-za-z]

how i rewrite identifier order number current definitions?

i don't know "second" match, i'm stranded here:

identifier {valid}[{valid}{digit}]* // syntax incorrect

thanks.

edit: whole exam way i'm using: .