-is not known. The condition may be lookahead or lookbehind (the condition
-is true if the lookahead or lookbehind is true), a (?{...}) construct (the
-condition is true if the code evaluates to a true value), or a number (the
-condition is true if the set of capturing parentheses named by the number
-matched).
+is not known. The condition must be one of the following:
+
+ (1) (2) ... true if 1st, 2nd, etc., capture matched
+ (<NAME>) ('NAME') true if named capture matched
+ (?=...) (?<=...) true if subpattern matches
+ (?!...) (?<!...) true if subpattern fails to match
+ (?{ CODE }) true if code returns a true value
+ (R) true if evaluating inside recursion
+ (R1) (R2) ... true if directly inside capture group 1, 2, etc.
+ (R&NAME) true if directly inside named capture
+ (DEFINE) always false; for defining named subpatterns