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Unit regex

This library unit provides support for regular expressions. The flavor depends on the particular installation platform:

• On UNIX systems that have PCRE (the Perl Compatible Regular Expression package) installed, PCRE is used.
• If PCRE is not available, and the C library provides regular expressions, these are used instead.
• on Windows (or of PCRE and libc regexes are not available), Dorai Sitaram's portable pregexp library is used.

grep

[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-search regex x)) lst) )

glob->regexp

[procedure] (glob->regexp PATTERN)

Converts the file-pattern PATTERN into a regular expression.

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

regexp

[procedure] (regexp STRING [IGNORECASE [IGNORESPACE [UTF8]]])

Returns a precompiled regular expression object for string. The optional arguments IGNORECASE, IGNORESPACE and UTF8 specify whether the regular expression should be matched with case- or whitespace-differences ignored, or whether the string should be treated as containing UTF-8 encoded characters, respectively.

Notes:

• regex doesn't allow (?: ) cloisters (non-capturing groups). Currently this means if you use utf8 matching, individual "." matching will return extra submatches.
• pregexp doesn't allow a # comment w/o a trailing newline.

regexp?

[procedure] (regexp? X)

Returns #t if X is a precompiled regular expression, or #f otherwise.

string-match

string-match-positions

[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). Note that the exact string is matched. For searching a pattern inside a string, see below. Note also that string-match is implemented by calling string-search with the regular expression wrapped in ^ ... $.

string-search

string-search-positions

[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.

string-split-fields

[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.

(define s "this is a string 1, 2, 3,")

(string-split-fields "[^ ]+" s)

  => ("this" "is" "a" "string" "1," "2," "3,")

(string-split-fields " " s #:infix)

  => ("this" "is" "a" "string" "1," "2," "3,")

(string-split-fields "," s #:suffix))
 
  => ("this is a string 1" " 2" " 3")

string-substitute

[procedure] (string-substitute REGEXP SUBST STRING [MODE])

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 MODE 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)"

string-substitute*

[procedure] (string-substitute* STRING SMAP [MODE])

Substitutes elements of STRING with string-substitute according to SMAP. SMAP should be an association-list where each element of the list is a pair of the form (MATCH . REPLACEMENT). Every occurrence of the regular expression MATCH in STRING will be replaced by the string REPLACEMENT

(string-substitute* "<h1>Hello, world!</h1>"
                    '(("<[/A-Za-z0-9]+>" . ""))))

=>  "Hello, world!"

regexp-escape

[procedure] (regexp-escape STRING)

Escapes all special characters in STRING with \, so that the string can be embedded into a regular expression.

(regexp-escape "^[0-9]+:.*$")
=>  "\\^\\[0-9\\]\\+:.\n.\\*\\$"

Platform-specific notes:

• due to a bug in the pregexp library, character classes enclosed in [ ... ] may not begin with a hyphen (-). A workaround is either to precede the hyphen with a backslash or use the range ---.

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