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Added example for Terminal priority setting #1504

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Added example for Terminal priority setting #1504

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17 changes: 16 additions & 1 deletion docs/grammar.md
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Expand Up @@ -102,10 +102,25 @@ num_list: "[" _separated{NUMBER, ","} "]" // Will match "[1, 2, 3]" etc.
### Priority

Terminals can be assigned a priority to influence lexing. Terminal priorities
are signed integers with a default value of 0.
are signed integers with a default value of 0. To give a terminal a priority other than 0, append `.p` to the end of the terminal name, whre `p` is the priority.

When using a lexer, the highest priority terminals are always matched first.

For example, the grammar:
```
start: AB | A_OR_B_PLUS
AB: "ab"
A_OR_B_PLUS: /[ab]+/
```
will parse `"ab"` as the token `AB` as written.

Giving `A_OR_B_PLUS` a higher priority will result in the same string being parsed as that token:
```
start: AB | A_OR_B_PLUS
AB: "ab"
A_OR_B_PLUS.1: /[ab]+/
```

When using Earley's dynamic lexing, terminal priorities are used to prefer
certain lexings and resolve ambiguity.

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