$Email::Address::addr_spec
This regular expression defined what an email address is allowed to look like.
(?-xism:(?-xism:(?-xism:(?-xism:(?-xism:\s*\((?:\s*(?-xism:(?-xism:(?>
+[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|(?-xism:\s*\((?:\s*(?-xism
+:(?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|))*\s*\)\s*)))
+*\s*\)\s*)|\s+)*(?-xism:[^\x00-\x1F\x7F()<>\[\]:;@\\,."\s]+(?:\.[^\x0
+0-\x1F\x7F()<>\[\]:;@\\,."\s]+)*)(?-xism:(?-xism:\s*\((?:\s*(?-xism:(
+?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|(?-xism:\s*\((?:
+\s*(?-xism:(?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|))*\
+s*\)\s*)))*\s*\)\s*)|\s+)*)|(?-xism:(?-xism:(?-xism:\s*\((?:\s*(?-xis
+m:(?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|(?-xism:\s*\(
+(?:\s*(?-xism:(?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|)
+)*\s*\)\s*)))*\s*\)\s*)|\s+)*"(?-xism:(?-xism:[^\\"])|(?-xism:\\(?-xi
+sm:[^\x0A\x0D])))+"(?-xism:(?-xism:\s*\((?:\s*(?-xism:(?-xism:(?>[^()
+\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|(?-xism:\s*\((?:\s*(?-xism:(?-
+xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|))*\s*\)\s*)))*\s*
+\)\s*)|\s+)*))\@(?-xism:(?-xism:(?-xism:(?-xism:\s*\((?:\s*(?-xism:(?
+-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|(?-xism:\s*\((?:\
+s*(?-xism:(?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|))*\s
+*\)\s*)))*\s*\)\s*)|\s+)*(?-xism:[^\x00-\x1F\x7F()<>\[\]:;@\\,."\s]+(
+?:\.[^\x00-\x1F\x7F()<>\[\]:;@\\,."\s]+)*)(?-xism:(?-xism:\s*\((?:\s*
+(?-xism:(?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|(?-xism
+:\s*\((?:\s*(?-xism:(?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0
+D]))|))*\s*\)\s*)))*\s*\)\s*)|\s+)*)|(?-xism:(?-xism:(?-xism:\s*\((?:
+\s*(?-xism:(?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|(?-x
+ism:\s*\((?:\s*(?-xism:(?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A
+\x0D]))|))*\s*\)\s*)))*\s*\)\s*)|\s+)*\[(?:\s*(?-xism:(?-xism:[^\[\]\
+\])|(?-xism:\\(?-xism:[^\x0A\x0D]))))*\s*\](?-xism:(?-xism:\s*\((?:\s
+*(?-xism:(?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x0D]))|(?-xis
+m:\s*\((?:\s*(?-xism:(?-xism:(?>[^()\\]+))|(?-xism:\\(?-xism:[^\x0A\x
+0D]))|))*\s*\)\s*)))*\s*\)\s*)|\s+)*)))
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