Negation

Anything that can be compiled to a lookaround, word boundary, or a character set can be negated.

Syntax

let Negation = '!' FixExpression;

See FixExpression.

Example

let no_boundary = !%;
!no_boundary (!>> !'a')

Support

Negation is supported in all flavors.

Behavior

The following kinds of expression can be negated:

• Word boundary %
• Lookarounds <<, >>
• Character set [...]
• Strings with exactly one code point
• Negations

Negation happens late in the compilation process, after variable and range expansion, and some optimizations. It unwraps non-capturing groups with only one element.

Arbitrary nesting is allowed; !!x is equivalent to x, if x is negatable.

Compilation

Negated word boundaries are compiled to \B. In JavaScript, this requires that Unicode is disabled. Negative lookbehind is compiled to (?<!...), negative lookahead is compiled to (?!...). Negative character sets are compiled to [^...]. When a character set contains exactly one shorthand, we try to just negate the shorthand to remove the character set; for example, ![s] can be compiled to \S.

Pomsky requires that a character set is not empty. [] is rejected at the syntax level. Another way to create an empty character set is to negate a full character set, e.g. ![s !s]. This must therefore forbidden.

Issues

Detecting full character sets is not yet implemented properly. The current implementation only rejects some common cases, like ![w !w], but fails to reject ![w !d], for example.

Negation currently does not work for alternations like !('a' | 'c').

History

• Negation changed to a late compilation step in Pomsky 0.11
  • ! syntactically allowed everywhere
  • resolved after variable and range expansion
  • can unwrap groups and turn single-char strings into character sets
  • arbitrary nesting of negations
• Initial implementation in Pomsky 0.1
  • ! syntactically only allowed before %, <<, >>, and [
  • no double negation