Problem

As described in #1816, there are cases when an usual search just isn't good enough.

Solution

Add a checkbox to turn the search in a Regex search

• Add a checkbox field near the term input to turn the search in a Regex
• If the checkbox is selected, make a regex search to the db

Testing Instructions

1. Open translation table with many translation strings, like a copy of WordPress core.
2. Try searching with the Regex option, with examples like the below, with and without case sensitive:

• ^(Activ|Disabl|Enab) - Will find any string starting with Activ..., Disabl... or Enabl...
• ^(activ\w*\s.*) - Will find any string starting with Activ... folowed by any other words, like Activate %s

Screenshots or screencast

Regex Search for ^(Activ|Disabl|Enab)

Regex Search for ^(activ\w*\s.*)

Fixes #1816