Recently I had to work with regex and wasn't sure if I was going to get the output I want based on the regex I came up with/ the solutions I googled and found. This is a good site that I chanced upon where I can paste the string that I would like to apply the regex on and look at the output to see if I can get what I want: https://regex101.com/.
For example, you have a string that says "TOTAL $2,000". You would like to extract the figures from that string using regex. You can input "TOTAL $2,000" within the box under TEST STRING and check if your regex is correctly extracting the figures as you want, which in this case, it is TOTAL \$\s*([^\n\r]*).