Linux
grep builder
Build a grep command with an explanation for each option and try your pattern on sample text.
What does this mean?
grep searches for text in files or in the output of other commands. Choose your options and you get the command with an explanation. You can try your pattern right away on a piece of sample text, so you can see what matches.
For testing patterns in general, you have the Regex tester. Looking for files by name? Use the find builder.