Url validator accept as valid an url like the following:
http://testsub+domanin.domanin.com:8080/test

but does not accept
http://test_sub_domanin.domanin.com:8080/test

The problem seems to be here:
symfony / src / Symfony / Component / Validator / Constraints / UrlValidator.php
Line 28: ([\pL\pN\pS-]+.)+[\pL]+ # a domain name

The fix can be as simple as replacing \pS (mathematical symbol) with \pP (punctuation character) or even better i think \pPc (Connector Punctuation character)