Some users are experiencing slow downs/timeouts caused by convert_urls_into_links() See
MDL-21168 for previous discussion.
A machine running php 5.2.1, pcre 6.6 and missing the 2 pcre configuration directives is not working.
A machine with php 5.2.4 and pcre 7.4 with the directives is working. I've got php 5.2.10 and pcre 7.8 with the directives and its working.
Theoretically pcre 6.6 or the missing configuration directives could be the source of the problem.
Those directives were introduced in php 5.2.0 so they should be there on the php 5.2.1 machine. That said, if they're missing I would have assumed sensible defaults would have been used.
The workaround described here (http://tracker.moodle.org/browse/MDL-21168?focusedCommentId=79154&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#action_79154) makes this work but the non-unicode modifier regexes fail the 2 unit tests to do with detecting urls containing non-ascii characters ie utf 8 characters.