Short answer: grep defaults to Basic Regular Expressions (BRE), but unescaped () and | are part of Extended Regular Expressions (ERE). ... <看更多>

GNU grep understands these with -P , but if you want portability, POSIX basic and extended regular expressions are more often used with Unix command line tools. ... <看更多>

-G option can be used to specify explicitly that BRE is needed; -E option will enable Extended Regular Expression (ERE). in GNU grep , BRE and ERE only ... ... <看更多>