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FilterRules

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FilterRules[rules, pattern]

gives those rules that have a left side that matches pattern.

FilterRules[rules, {$pattern_1$, $pattern_2$, ...}]

gives those rules that have a left side that match at least one of $pattern_1$, $pattern_2$, ...

• • FilterRules[{x -> 100, y -> 1000}, x]

• • FilterRules[{x -> 100, y -> 1000, z -> 10000}, {a, b, x, z}]