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RE2
In short and in plain language
RE2 is a toolkit for search patterns in text.
Some tools for search queries use its way of writing. You write filters in that form there.
Some patterns from other toolkits do not work there.
Definition
RE2 is a widely used software library for regular expressions, that is, search patterns in text. Some tools for analysing search queries expect filters in its syntax. Some patterns from other regex flavours do not work there.