Example of Regular expression on documentation not work on MantisBT.
On section "6.8.3 Manage Custom Fields" of administrator documentation says:

All fields are also compared against the regular expression. If the value matches the expression, then the

value is stored. For example, the expression "/^-?([0-9])*$/" can be used to constrain an integer.

SYSTEM WARNING

'preg_match(): Unknown modifier '^'' in 'C:\wamp\www\mantisbt_210dev\core\custom_field_api.php' line 1084

Maybe is a good idea change the example of regular expression in documentation and put another valid example.