automata: fix bug where reverse NFA lacked an unanchored prefix #1170
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Previously, when compiling a Thompson NFA, we were omitting an unanchored prefix when the HIR contained a `^` in its prefix. We did this because unanchored prefix in that case would never match because of the requirement imposed by `^`. The problem with that is it's incorrect when compiling a reverse automaton. For example, in the case of building a reverse NFA for `^Qu`, we should sitll include an unanchored prefix because the `^` in that case has no conflict with it. It would be like if we omitted an unanchored prefix for `Qu$` in a forward NFA, which is obviously wrong. The fix here is pretty simple: in the reverse case, check for `$` in the suffix of the HIR rather than a `^` in the prefix. Fixes #1169
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Previously, when compiling a Thompson NFA, we were omitting an
unanchored prefix when the HIR contained a
^in its prefix. We didthis because unanchored prefix in that case would never match because of
the requirement imposed by
^.The problem with that is it's incorrect when compiling a reverse
automaton. For example, in the case of building a reverse NFA for
^Qu,we should sitll include an unanchored prefix because the
^in thatcase has no conflict with it. It would be like if we omitted an
unanchored prefix for
Qu$in a forward NFA, which is obviously wrong.The fix here is pretty simple: in the reverse case, check for
$in thesuffix of the HIR rather than a
^in the prefix.Fixes #1169