The "Single character alternation" warning shows up in JetBrains' IDEs. The warning reports that the single character alternation (such as a|b|c) in a regular expression would be better written as a character class instead (i.e. [abc]). This might even be slightly more performant when it's matching.
For example, consider the following regular expression that allows one of the symbols from; =, :, - or _:
=|:|-|_
This could be re-written as:
[=|:|\-|_]
Please keep in mind that in character classes, the characters ^-]\ need to be escaped.
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