Expressions lets you play with regular expressions (regex). Just type your pattern and some text to test it. All the changes are updated while you're typing text or pattern. App highlights the syntax, so the pattern is easy-to-read and modify. If you use subexpressions (groups), you can highlight them by pressing Cmd+Shift+] (or [ - for decreasing the highlighted group number). If you don't remember the regex syntax, just press Cmd+r or click button in the top-right. You will get the reference sheet, so you don't have to remember everything.
The user interface is very minimalistic and non-distractive. By default the color is the same as your menubar, but you can always switch to dark or light mode (see: Preferences). You can also open app in beautiful full screen mode which is perfect when you want to focus or to demonstrate regular expressions to other people. Note:Expressions uses ICU regular expression standard. More information here.