regex lookahead but ignore if specific char is found

so I have this text

onmousedown=
on mousedown=

I want to identify all "on" words that are succeded by a "=", but not if there is a space between the "on" word and the "="
=> line 1 should be a match (on)
=> line 2 should not be a match

>Solution :

This should work for you:

^on[^ ]+=$

it matches on followed by non-whitespace characters followed by =