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\Wmeans any character other than [a-zA-Z0-9_] which includes\nas well.grepyou can choose one of the four major regexp dialects (keys-E,-F,-G,-P). And even then, by using different versions ofgrepyou can encounter differences in regexp processing. Yourgrepin "extended" mode (you have-Ekey in the command) does not include\nin\W. The online regexp checker most likely using javascript or Perl versions - they do include it. Run your command with-Pinstead of-Eand see the difference.\nas pointed out by @Stéphane Chazelas. Because I tried a similar thing for\tand it worked. eg -echo $'a\tb\nc\td' | grep -zEo 'c[^;]*d'outputsc[8 spaces]dbutecho $'a\tb\nc\td' | grep -zEo 'a[^;]*d'outputs nothing.grep: "a\tb" and "c\td". So it can find pattern which includes "c" and "d", but letters "a" and "d" are on different lines - so nothing is found.-zflag so it doesn't treat the input as two separate lines. eg -echo $'a\nb' | grep -z '\n' | lessthe output isa b ^@(each in a new line).^@represents null character. The input is treated as null terminated string. So\nmatches a part of line and grep prints the matching line (the whole line itself).