In addition to the primitives string? and string-null?,
which are already in the Guile core, the string predicates
string-any and string-every are defined by SRFI-13.
— Scheme Procedure: string-any pred s [start [end]]
Check if the predicate pred is true for any character in
the string s, proceeding from left (index start) to
right (index end). Return the value produced by the first
successful application of pred, or #f.
— Scheme Procedure: string-every pred s [start [end]]
Check if the predicate pred is true for every character
in the string s, proceeding from left (index start)
to right (index end). Return the value produced by the
successful application of pred to the final character in
the range, or #f.