Alternation

An alternation matches one of several alternatives.

Syntax

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

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

See Intersection.

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)+.

Alternations are subject to advanced optimizations:

• Merging of single-character alternations: 'a' | ['bc'] becomes [a-c].
• Prefix merging: 'world' | 'wow' becomes wo(?:rld|w).
• Empty alternatives: If the first or last alternative is empty, it is replaced with a ? or ?? quantifier.

These alternatives are sometimes combined. There is no guarantee regarding in which order the optimizations are applied.

Issues

Merging of alternatives is currently not supported if they are not adjacent.

History

• Many optimizations added in Pomsky 0.12
• Support for leading pipes added in Pomsky 0.6
• Initial implementation in Pomsky 0.1