expressions. The method "prefix" on a RegExp behaves exactly the same
as the "exec" method except it returns "undefined" if the match failed
because there was an insufficient number of characters in the
input. E.g.
/^foo/.prefix("foo") => ["foo"] (just like exec)
/^foo/.prefix("fox") => null (just like exec)
/^foo/.prefix("fo") => undefined (whereas exec returns null)
git-svn-id: svn://10.0.0.236/trunk@85760 18797224-902f-48f8-a5cc-f745e15eee43
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