Are you looking for a tool to run regular expression searches or to help develop & test the regex?
Cross Platform Regular Expression Tool
I am a big fan of Robert Kern's python grin tool/library.
Once you have python installed on your system just run:
pip install grin3 or for python 2 installations leave out the 3
What it does
On my side I always use this Chrome App.
It is very simple and light and it support the few things I need (multiline, highlight and replace). The big plus against the online tool regex101 is that once installed on your chrome browser, you can use it without need of Internet connections.
When using an IDE, you cal also install plugins into:
IntelliJ IDEA: ...