E4218

Compiler diagnostic name: lexscan_pattern_not_anchored.

A lexical regex pattern is not anchored where anchoring is required.

Patterns used by streaming lexscan, or by either expression with the longest strategy, must start at the current input position. Anchor each such regex with ^.

Erroneous example

///|
pub fn classify(input : String) -> String {
  lexmatch input with longest {
    re"[a-z]+" => "word"
    _ => "other"
  }
}

Suggestion

Add ^ to the start of the regex. Add $ as well only when the case must consume the whole input.

///|
pub fn classify(input : String) -> String {
  lexmatch input with longest {
    (re"^[a-z]+", after=_) => "word"
    _ => "other"
  }
}