default: input=XabcY region=[1,4) group=abc span=[1,4)
bounds anchoring=false transparent=false: anchors=false lookaround=false
bounds anchoring=false transparent=true: anchors=false lookaround=true
bounds anchoring=true transparent=false: anchors=true lookaround=false
bounds anchoring=true transparent=true: anchors=true lookaround=true
region=abcYY: matches=false lookingAt=true find=true
reset: region=[0,12) anchoring=false transparent=true group=undefined
find(7): region=[0,12) group=def span=[7,10)
empty-region=matched; invalid-regions=3 rejected; invalid-find-start=1 rejected
PASS 39 checks; fixed ASCII input, no regex performance claim
