Url character length limitation?

No, there is no URI length limit in BIG-IP 9.x. Additionally, there are more efficient ways to perform case-insensitive comparisons. I don't think you need the regex, so you could do something like this:

 
 if {[string compare -nocase -length 21 [HTTP::uri] "/servlet/store/store?"] == 0} { 
     pool Store-Pool 
 }