Alternation

An alternation matches one of several alternatives.

Syntax 

let Alternation = ('|'? Alternatives)?;

let Alternatives = Alternative ('|' Alternative)*;

let Alternative = FixExpression+;

See FixExpression.

Note that an alternation may have a leading pipe. Also note that an alternative may not be empty, i.e. | | is not allowed. Use an empty string instead, e.g. 'foo' | '' | 'bar'.

Example 

| 'hello'
| 'pomsky'+

Support

Alternation is supported in all flavors.

Behavior

Alternatives are matched consecutively. The first alternative that matches is used.

Compilation

Compiled to an alternation. The example above would be compiled to hello|(?:pomsky)+.

Issues

Alternations aren’t yet properly optimized. Planned optimizations include:

  • Common prefixes: 'test' | 'testament' | 'testing' to test(?:|ament|ing)
  • Single char alternation: 'a' | 'c' | '?' to [ac?]

History

  • Support for leading pipes added in Pomsky 0.6
  • Initial implementation in Pomsky 0.1
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