Rules

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

Our current grammar doesn't handle whitespace:

$ ./calc --target=parse
> 12 + 45
Failed to parse source

Whitespace in regexes is metasyntactic, and ignored by default

The 'rule' keyword says that whitespace in the regex corresponds to whitespace in the input

rule TOP { <integer> <addop> <integer> {*} }