One of my projects involved validating only the query part of the url using regular expressions (i.e. everything after the ?). It took me ages to figure out a proper regex, so I thought it might help someone here by posting it:

^(?:[a-zA-Z%0-9+]+=[a-zA-Z%0-9+]+(?!=)&)*[a-zA-Z%0-9+]+=[a-zA-Z%0-9+]+(?!.)

Note: this only works for validating the query string, so adding the actual url will prevent a match, making the string to search invalid.