巷子 is the colloquial form; 巷 alone is more literary. In Beijing, alleys are called 胡同 (hútòng); in southern China, 弄堂 (lòngtáng). 巷子 is the general term understood everywhere.
culture
The saying 酒香不怕巷子深 (jiǔ xiāng bù pà xiàngzi shēn) means 'good wine fears no deep alley' - quality speaks for itself.
