Version: 4.2.1

9.5 Quantifiers

The quantifiers *, +, and ? match respectively: zero or more, one or more, and zero or one instances of the preceding subpattern.

  > (regexp-match-positions #rx"c[ad]*r" "cadaddadddr")

  ((0 . 11))

  > (regexp-match-positions #rx"c[ad]*r" "cr")

  ((0 . 2))

  > (regexp-match-positions #rx"c[ad]+r" "cadaddadddr")

  ((0 . 11))

  > (regexp-match-positions #rx"c[ad]+r" "cr")

  #f

  > (regexp-match-positions #rx"c[ad]?r" "cadaddadddr")

  #f

  > (regexp-match-positions #rx"c[ad]?r" "cr")

  ((0 . 2))

  > (regexp-match-positions #rx"c[ad]?r" "car")

  ((0 . 3))

In #px syntax, you can use braces to specify much finer-tuned quantification than is possible with *, +, ?:

It is evident that + and ? are abbreviations for {1,} and {0,1} respectively, and * abbreviates {,}, which is the same as {0,}.

  > (regexp-match #px"[aeiou]{3}" "vacuous")

  ("uou")

  > (regexp-match #px"[aeiou]{3}" "evolve")

  #f

  > (regexp-match #px"[aeiou]{2,3}" "evolve")

  #f

  > (regexp-match #px"[aeiou]{2,3}" "zeugma")

  ("eu")

The quantifiers described so far are all greedy: they match the maximal number of instances that would still lead to an overall match for the full pattern.

  > (regexp-match #rx"<.*>" "<tag1> <tag2> <tag3>")

  ("<tag1> <tag2> <tag3>")

To make these quantifiers non-greedy, append a ? to them. Non-greedy quantifiers match the minimal number of instances needed to ensure an overall match.

  > (regexp-match #rx"<.*?>" "<tag1> <tag2> <tag3>")

  ("<tag1>")

The non-greedy quantifiers are respectively: *?, +?, ??, {m}?, {m,n}?. Note the two uses of the metacharacter ?.