Hello,
In the 3.7.7 documentation on Regular Expression, the Conditional Construct, (?(id/name)yes-pattern|no-pattern), is discussed. (This is a very thorough document, by the way. Good job!)
One example given for the Conditional Construct does not work as described. Specifically, the example gives this matching pattern '(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)' and states that it will NOT MATCH the string '<user@host.com'. In my tests the pattern DOES MATCH that string.
(The other examples work as described.) 
This may be a bug in re since it seems to me that the match should fail.
Respectfully,
Leon