RUM JavaScript API - 1.345.1
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    Variable VALID_KEY_REGEXConst

    VALID_KEY_REGEX: RegExp = ...

    Validates custom field key names.

    Pattern: /^[\dA-Za-z]+(?:\.[A-Za-z][\dA-Za-z]*|_[\dA-Za-z]+)*$/

    Rules: alphanumeric characters (a-z, A-Z, 0-9), underscores (_) and dots (.). Each dot must be followed by an alphabetic character. Each underscore must be followed by an alphanumeric character.

    valid:   `"my_key"`, `"event.type"`, `"my.custom2_key"`
    
    invalid: `"my-key"`, `"my..key"`, `"_key"`, `".key"`