Lookahead groups #145
Lookahead groups #145
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Tests kept to be used for the replacement syntax
Also adds test coverage for syntax errors in groups
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Oh and I also found some bugs where wrong grammar syntax in negation or captures could lead to the grammar building successfully but then failing during runtime with a nil dereference error. |
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Probably review by commits |
| @@ -155,7 +155,8 @@ The grammar format is: | |||
| - `<expr> <expr> ...` Match expressions. | |||
| - `<expr> | <expr> | ...` Match one of the alternatives. Each alternative is tried in order, with backtracking. | |||
| - `!<expr>` Match any token that is _not_ the start of the expression (eg: `@!";"` matches anything but the `;` character into the field). | |||
| - `!?<expr>` Negative lookahead - the sequence will not match if this expression matches (lookahead won't be consumed) | |||
| - `(?= ... )` Positive lookahead group - requires the contents to match further input, without consuming it | |||
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Maybe move these up under group, though not a big deal.
| @@ -0,0 +1,40 @@ | |||
| package participle_test | |||
| Int int `| @Int` | ||
| } | ||
| type op struct { | ||
| Op string `@('+' | '*' (?= @Int))` |
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Very nice, this will be super useful.
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To be honest, I couldn't actually find a good example of what to use positive lookahead for, the best I could quickly come up with was this rather silly test. But it was easy to have positive lookahead supported when I was refactoring negative lookahead anyway, so I thought why not.
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Yeah good point, with Participle's backtracking it's not super useful...
Partially reverts my changes from #144 to change the negative lookahead syntax from
!? <expr>to(?! <expr> ...), in order to have syntax consistent with PERL-like regular expressions. Also adds support for positive lookahead groups, because why not.Relates to #134