Bug:
There may be more than one "(" or ")" in regular expression used for dimension extraction although there is only one capturing group. Examples include non-capturing groups, modificators like "(?i)", and escaped character sequences like "(". Regular expressions with more than one "(" or ")" are rejected currently.

Example:
(?i)example.com/(?:SOME_PART/)?([a-z]{2})(?:$|/)
This is captures exactly one group, yet is not allowed. (This is what I wanted to use in my project, actually.)

Root cause:
Counting the number of "(" or ")" as is done in https://github.com/piwik/plugin-CustomDimensions/blob/master/Dimension/Extraction.php#L46 is just wrong: It does not ensure that there is exactly one capturing group.

Better solutions:
Short term: Remove this useless test. Add an example or warning regarding the number of groups.
Long term: Include "live-preview" of matches as described in another bug report. Warn when more than one group captures.