I found [this](http://stackoverflow.com/questions/2497294/regular-expression-to-validate-a-google-analytics-ua-number) on StackOverflow.

And `/UA-[0-9]+-[0-9]+/i` rules out accounts like `UA-287932` which is still valid (according to one of the comment on SO). So probably better would be `/UA-[0-9-]+[0-9]+/i`.