A more relevant example

From the "Python Language Reference Manual" (Release 2.3):

identifier ::= (letter | "_") (letter | digit | "_")*
letter ::= lowercase | uppercase
lowercase ::= "a"..."z"
uppercase ::= "A"..."Z"
digit ::= "0"..."9"

In PGE, we can write:

token identifier { [ <alpha> | '_' ] \w* }

PGE already has built-in definitions for <alpha>, <upper>, <lower>, <digit>, etc. (Furthermore, they're Unicode aware!)