Provided herein are systems and methods for identifying personal identifiers in content. An entity engine may receive content to identify candidate personal identifiers. The entity engine may determine that a text string in the content matches to a data format specified in entity definitions corresponding to types of personal identifiers and a rule for finding a geographic or linguistic term in the content correlated to the specific type of personal identifier. Each entity definition may specify a data format for finding a specific type of personal identifier in content. The data format corresponds to a type of personal identifier. The entity engine may identify, according to a rule of the first entity definition, a geographic or linguistic term in the content correlated to the type of personal identifier. The entity engine may classify the text string as the type of personal identifier, for preventing data breach or exfiltration.