chickadee » srfi-130 » string-index

string-index searches through s from the left, returning the cursor of the first occurrence of a character which satisfies the predicate pred. If no match is found, it returns end. string-index-right searches through s from the right, returning the cursor of the successor of the first occurrence of a character which satisfies the predicate pred. If no match is found, it returns start.

The start and end parameters specify the beginning and end cursors or indexes of the search; the search includes the start, but not the end. Be careful of "fencepost" considerations: when searching right-to-left, the first position considered is (string-cursor-prev end), whereas when searching left-to-right, the first index considered is start. That is, the start / end indexes describe the same half-open interval [start, end) in these procedures that they do in all the other SRFI 130 procedures.

The skip functions are similar, but use the complement of the criteria: they search for the first char that doesn't satisfy pred. E.g., to skip over initial whitespace, say

(substring/cursors s (string-skip s char-whitespace?))

Note that the result is always a cursor, even when start and end are indexes. Use string-cursor->index to convert the result to an index. Therefore, these four functions are not entirely compatible with their SRFI 13 counterparts, which return #f on failure.

These functions can be trivially composed with string-take and string-drop to produce take-while, drop-while, span, and break procedures without loss of efficiency.