Fix trailing space issue

While this fixes the specific RegEx so that you can have spaces at the end, I wasn't able to replicated your exact problem.  Getpot strips the space for you so I'm no exactly sure how this tripped you up.  I tried several variations of spacing after the filename with both single and double quotes and everything worked as expected.  

I tested this particular regular expression by hardcoding a few examples right above and making sure that they still functioned as intended.

closes #3810