When working with regular expressions, it can be difficult to understand why a particualar pattern is not working as you would expect.
The visual debug tool at debuggex.com makes it much easier to step through and actually visualize how a pattern is working or, as is often the case, not working.
Here is an overview of how it works.
magp.ie 
Hi, I also suggest https://extendsclass.com/regex-tester.html as visual debug tool