表达式 is the technical 'expression' - math, programming, logic. For 'expression' in the sense of facial look, use
表情; for verbal phrasing,
说法 or
表达 the verb. Common collocates:
正则表达式 (regex),
代数表达式 (algebraic expression),
布尔表达式 (Boolean expression), Lambda
表达式.
