In frontend (Administration->General--> Regular expressions) global expression subexpressions are displayed sorted by type. So, display order for subexpressions of the same type is not defined.
In agent active checks (e.g. log[] and logrt[] using global expressions) order of subexpression evaluation is not defined. Although mathematically the order of evaluation is irrelevant (results of individual subexpression evaluations are AND'ed), it would be desirable to ensure that subexpressions are evaluated in the same order as user sees them in frontend. Then user could optimize subexpression order starting from most-likely-to-match.