如何在grep shell命令中使用正则表达式

Question

I am not able to use [-]+[\\n][-]+ this regex in shell. Can somebody help to use it with escape characters in grep. I want to find lines like '----\\n----' from the file

Answer 1

Plain old grep uses Basic Regular Expression so you don't have immediate access to the + quantifier. In modern (POSIX) grep you can use this functionality, but it requires a backslash before the quantifier, like \\+ :

grep '-\+\\n-\+' file

(notice also the single quotes to prevent the shell from doing its own backslash substitution; and pay attention to how n matches just itself and a literal backslash is matched either with a double \\\\ to escape it or in a character class [\\] ); or you can use grep -E (aka egrep ) which supports Extended Regular Expression :

grep -E '-+\\n-+' file

For the record, [\\n] matches a single character which can be either a literal backslash or a literal n . This construct is called a character class; inexplicably, newcomers seem to want to try to use square brackets for pretty much anything just because they "look like regex", apparently.