Backtracking control -- tokens

regex TOP     { <integer> <addop> <integer> {*} }
regex integer { \d+ {*} }
regex addop   { [ '+' | '-' ] {*} }

The use of \d+ can be very inefficient

Consider "1873649872934897234009x42" as an input string

A colon after an expression prevents backtracking into the expression

regex integer { \d+: {*} }