表达式

表达式 is the technical 'expression' — math, programming, logic. For 'expression' in the sense of facial look, use 表情 (biǎoqíng); for verbal phrasing, 说法 (shuōfǎ) or 表达 (biǎodá) 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.

memory

Reads as 表达 (biǎodá) 'to express' + 式 (shì) 'form.' Same suffix as 公式 (gōngshì, 'formula') and 等式 (děngshì, 'equation') — anything formal-mathematical takes 式.