"rule" but not when about rust-peg HOT 2 CLOSED

Just express what you say in natural language in PEG DSL:

rule raw_identifier() -> &'input str
  = !"test" i:$(['a'..='z'|'A'..='Z'|'0'..='9'|'_']*) { i }

from rust-peg.

Unlike regex, PEGs backtrack only on match failures in a loop or ordered choice, so once the raw_identifier()* matches the second test, the parser will be at the end of input and "test" will fail.

As @Mingun said, you can use the negative lookahead ! operator for this: !"test" will fail to match if "test" would match at the current position, or match and consume no input if "test" would not match.

Also, might be just how you simplified your code for the issue, but it looks like your examples would fail because nothing would match the spaces.

from rust-peg.