From my test site hosted on Dreamhost I tried to save a topic with a RegExp instruction:
Every time I get a 503 error on save:
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
Here on ~develop I can save the topic....
What does work is fooling the system:
Guessing from reports such as http://wordpress.org/support/topic/63283
this could be related to mod_security or the configuration of it at Dreamhost?
Yes, going to https://panel.dreamhost.com/index.cgi?tree=domain.manage&
and Uncheck "Extra Web Security?" under PHP did the trick.