tl;dr

Don't forget require 'English' if you use a named global such as $LAST_MATCH_INFO. Otherwise this could result in an annoying bug.

Background

The code snippet of How to fix gsub on SafeBuffer objects was adjusted during a rubocop upgrade.

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Rubocop tells you everything you need to know to fix the offense Style/SpecialGlobalVars:

Style/SpecialGlobalVars: Prefer $LAST_MATCH_INFO from the stdlib 'English' module (don't forget to require it) over $~.

In this scenario you should be very careful. If you forget to require 'English', your code may not work any longer as expected.

# Before
Thread.current[:LAST_MATCH_DATA] = $~

# After rubocop ugrade - was not working as expected
Thread.current[:LAST_MATCH_DATA] = $LAST_MATCH_INFO

# Fixed
require 'English'

Thread.current[:LAST_MATCH_DATA] = $LAST_MATCH_INFO