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David Majda 86769a6c5c Error handling: Make |?| return |null| on unsuccessful match
Before this commit, the |?| operator returned an empty string upon
unsuccessful match. This commit changes the returned value to |null|. It
also updates the PEG.js grammar and the example grammars, which used the
value returned by |?| quite often.

Returning |null| is possible because it no longer indicates a match
failure.

I expect that this change will simplify many real-world grammars, as an
empty string is almost never desirable as a return value (except some
lexer-level rules) and it is often translated into |null| or some other
value in action code.

Implements part of #198.
11 years ago
..
arithmetics.pegjs Text nodes: Use text nodes in examples/arithmetics.pegjs 12 years ago
css.pegjs Error handling: Make |?| return |null| on unsuccessful match 11 years ago
javascript.pegjs Error handling: Make |?| return |null| on unsuccessful match 11 years ago
json.pegjs Error handling: Use a special value (not |null|) to indicate failure 11 years ago