Totally minimalistic, non–distractive user interface. Dark and light mode. Pattern syntax highlighting. Beautiful full screen view. Pattern and text group highlighting. Regular expression reference sheet.

Expressions supports switching between pattern groups by pressing cmd+shift { and cmd+shift+}. Copying pattern can be escaped, so you can paste it directly into your code. And the other way round, of course (see Edit menu.) Thanks to "Allow whitespaces and comments" option, you can break pattern into parts which makes editing much simpler (see Preferences.)

Compatibility: macOS 10.11 or later, 64-bit processor

More info - https://itunes.apple.com/us/app/expressions/id913158085?mt=12