## 6.17 Unit: regex

[procedure] (grep REGEX LIST)
  Returns all items of `LIST` that match the regular expression `REGEX`. This procedure could be defined as follows:

      (define (grep regex lst)
        (filter (lambda (x) (string-match regex x)) lst) )

[procedure] (pattern`->`regexp PATTERN)
  Converts the file-pattern `PATTERN` into a regular expression.

      (pattern->regexp "foo.*") ==> "foo\..*"

[procedure] (regexp STRING)
  Returns a precompiled regular expression object for `string`.

[procedure] (regexp? X)
  Returns `#t` if `X` is a precompiled regular expression, or `#f` otherwise.

[procedure] (string-match REGEXP STRING [START
  )]
[procedure] (string-match-positions REGEXP STRING [START
  )] Matches the regular expression in `REGEXP` (a string or a precompiled regular expression) with `STRING` and returns either `#f` if the match failed, or a list of matching groups, where the first element is the complete match. If the optional argument `START` is supplied, it specifies the starting position in `STRING`. For each matching group the result-list contains either: `#f` for a non-matching but optional group; a list of start- and end-position of the match in `STRING` (in the case of `string-match-positions`); or the matching substring (in the case of `string-match`).

[procedure] (string-search REGEXP STRING [START [RANGE
  ])]
[procedure] (string-search-positions REGEXP STRING [START [RANGE
  ])] Searches for the first match of the regular expression in `REGEXP` with `STRING`. The search can be limited to `RANGE` characters. Otherwise the procedures have the same behaviour as `string-match` / `string-match-positions`.

[procedure] (string-split-fields REGEXP STRING [MODE [START]])
  Splits `STRING` into a list of fields according to `MODE`, where `MODE` can be the keyword `#:infix` (`REGEXP` matches field separator), the keyword `#:suffix` (`REGEXP` matches field terminator) or `#t` (`REGEXP` matches field), which is the default.

[procedure] (string-substitute REGEXP SUBST STRING [INDEX])
  Searches substrings in `STRING` that match `REGEXP` and substitutes them with the string `SUBST`. The substitution can contain references to subexpressions in `REGEXP` with the `
  NUM` notation, where `NUM` refers to the NUMth parenthesized expression. The optional argument `INDEX` defaults to 1 and specifies the number of the match to be substituted. Any non-numeric index specifies that all matches are to be substituted.

      (string-substitute "([0-9]+) (eggs|chicks)" "\\2 (\\1)" "99 eggs or 99 chicks" 2)
       ==> "99 eggs or chicks (99)"