with-input-from-string

(with-input-from-string (stream string &key index start end) body...)

Binds stream to an input stream reading from string, evaluates the body forms, and returns the value of the last one. read-line consumes the string line by line and returns nil at the end; read parses one datum and leaves the stream just after it, so successive calls walk the string datum by datum and the rest of a line after the first datum is not lost. Works in all four backends.

:start and :end bound the part of the string the stream reads. :index names a place that, when the body returns normally, receives the index in string of the first character the body did not read -- a character given back with unread-char counts as not read; a non-local exit leaves it untouched.

Naming the bound variable *standard-input* redirects the whole stream-argument-less read family for the extent of the body -- including inside called functions -- because read-line, read-char, read and peek-char read the current (dynamically bound) value of *standard-input* at call time. A nil stream argument is the same designator, so a reader forwarding its own optional argument follows the redirect too; only t always names the process standard input.