6.6 Searching Strings

The first few procedures in this section perform string search, in which a given string (the text) is searched to see if it contains another given string (the pattern) as a proper substring. At present these procedures are implemented using a hybrid strategy. For short patterns of less than 4 characters, the naive string-search algorithm is used. For longer patterns, the Boyer-Moore string-search algorithm is used.

procedure: string-search-forward pattern string

procedure: substring-search-forward pattern string start end

Pattern must be a string. Searches string for the leftmost occurrence of the substring pattern. If successful, the index of the first character of the matched substring is returned; otherwise, #f is returned.

substring-search-forward limits its search to the specified substring of string; string-search-forward searches all of string.

(string-search-forward "rat" "pirate")
    ⇒ 2
(string-search-forward "rat" "pirate rating")
    ⇒ 2
(substring-search-forward "rat" "pirate rating" 4 13)
    ⇒ 7
(substring-search-forward "rat" "pirate rating" 9 13)
    ⇒ #f

procedure: string-search-backward pattern string

procedure: substring-search-backward pattern string start end

Pattern must be a string. Searches string for the rightmost occurrence of the substring pattern. If successful, the index to the right of the last character of the matched substring is returned; otherwise, #f is returned.

substring-search-backward limits its search to the specified substring of string; string-search-backward searches all of string.

(string-search-backward "rat" "pirate")
    ⇒ 5
(string-search-backward "rat" "pirate rating")
    ⇒ 10
(substring-search-backward "rat" "pirate rating" 1 8)
    ⇒ 5
(substring-search-backward "rat" "pirate rating" 9 13)
    ⇒ #f

procedure: string-search-all pattern string

procedure: substring-search-all pattern string start end

Pattern must be a string. Searches string to find all occurrences of the substring pattern. Returns a list of the occurrences; each element of the list is an index pointing to the first character of an occurrence.

substring-search-all limits its search to the specified substring of string; string-search-all searches all of string.

(string-search-all "rat" "pirate")
    ⇒ (2)
(string-search-all "rat" "pirate rating")
    ⇒ (2 7)
(substring-search-all "rat" "pirate rating" 4 13)
    ⇒ (7)
(substring-search-all "rat" "pirate rating" 9 13)
    ⇒ ()

procedure: substring? pattern string

Pattern must be a string. Searches string to see if it contains the substring pattern. Returns #t if pattern is a substring of string, otherwise returns #f.

(substring? "rat" "pirate")             ⇒  #t
(substring? "rat" "outrage")            ⇒  #f
(substring? "" any-string)              ⇒  #t
(if (substring? "moon" text)
    (process-lunar text)
    'no-moon)

procedure: string-find-next-char string char

procedure: substring-find-next-char string start end char

procedure: string-find-next-char-ci string char

procedure: substring-find-next-char-ci string start end char

Returns the index of the first occurrence of char in the string (substring); returns #f if char does not appear in the string. For the substring procedures, the index returned is relative to the entire string, not just the substring. The -ci procedures don’t distinguish uppercase and lowercase letters.

(string-find-next-char "Adam" #\A)              ⇒  0 
(substring-find-next-char "Adam" 1 4 #\A)       ⇒  #f
(substring-find-next-char-ci "Adam" 1 4 #\A)    ⇒  2 

procedure: string-find-next-char-in-set string char-set

procedure: substring-find-next-char-in-set string start end char-set

Returns the index of the first character in the string (or substring) that is also in char-set, or returns #f if none of the characters in char-set occur in string. For the substring procedure, only the substring is searched, but the index returned is relative to the entire string, not just the substring.

(string-find-next-char-in-set my-string char-set:alphabetic)
    ⇒  start position of the first word in my-string
; Can be used as a predicate:
(if (string-find-next-char-in-set my-string
                                  (char-set #\( #\) ))
    'contains-parentheses
    'no-parentheses)

procedure: string-find-previous-char string char

procedure: substring-find-previous-char string start end char

procedure: string-find-previous-char-ci string char

procedure: substring-find-previous-char-ci string start end char

Returns the index of the last occurrence of char in the string (substring); returns #f if char doesn’t appear in the string. For the substring procedures, the index returned is relative to the entire string, not just the substring. The -ci procedures don’t distinguish uppercase and lowercase letters.

procedure: string-find-previous-char-in-set string char-set

procedure: substring-find-previous-char-in-set string start end char-set

Returns the index of the last character in the string (substring) that is also in char-set. For the substring procedure, the index returned is relative to the entire string, not just the substring.