Expressions allows to play with regular expressions*. Just type your pattern and some text to test it. All the changes are updated while youre 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 dont remember the regex syntax, just press cmd+r or click button in the top-right. You will get the reference sheet, so you dont 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. * Expressions uses ICU regular expression standard. More information you will find here: https://developer.apple.com/reference/foundation/nsregularexpression