mod_rewrite to iRule conversion question from a newcomer

You're looking for parens there instead of square brackets. Like this:

when HTTP_REQUEST {
 if { not ( [HTTP::uri] ends_with ".*"  ) } {
  log local0. "url didn't match .*"
 }
} 
 

The iRules interpreter is attempting to execute the URI as a command because that's what it expects after a square bracket - that's why it fails. Parenthesis are how "order" is specified in iRules, curly-braces are how conditionals are separated, and square brackets identify TCL commands to execute.

You've got a little more work to do on your logic, but you're on the right track. Let me know if you need some additional help to get there.