A generalized boolean.
nil or a function that takes arguments like
precompile-regexp returns a precompiled regular expression object suitable for passing as pattern to functions like find-regexp-in-string.
case-sensitive controls whether string is precompiled as a case sensitive or case insensitive search. A non-nil value means a case sensitive pattern. The value
nil (the default) means a case insensitive pattern.
error-function is used when the string is not a legal regular expression. In this case, if error-function is not
nil, it is applied to a list of arguments which are designators for a condition like the arguments that
error takes. If error-function is
nil as the first argument and the list of arguments as a second return value, condition-designators. error-function defaults to
For the regular expression syntax, see find-regexp-in-string.
LispWorks User Guide and Reference Manual - 20 Sep 2017