grep regex with backtick matches all lines

Question

$ cat file
anna
amma
kklks
ksklaii

$ grep '\`' file
anna
amma
kklks
ksklaii

Why? How is that match working ?

Answer 1

This appears to be a GNU extension for regular expressions. The backtick ('\\`') anchor matches the very start of a subject string, which explains why it is matching all lines. OS X apparently doesn't implement the GNU extensions, which would explain why your example doesn't match any lines there. See http://www.regular-expressions.info/gnu.html

If you want to match an actual backtick when the GNU extensions are in effect, this works for me:

grep '[`]' file

Answer 2

twm's answer provides the crucial pointer, but note that it is the sequence \\` , not ` by itself that acts as the start-of-input anchor in GNU regexes .

Thus, to match a literal backtick in a regex specified as a single-quoted shell string , you don't need any escaping at all , neither with GNU grep nor with BSD/macOS grep :

$ { echo 'ab'; echo 'c`d'; } | grep '`'
c`d

When using double-quoted shell strings - which you should avoid for regexes , for reasons that will become obvious - things get more complicated, because you then must escape the ` for the shell's sake in order to pass it through as a literal to grep :

$ { echo 'ab'; echo 'c`d'; } | grep "\`"
c`d

Note that, after the shell has parsed the "..." string, grep still only sees ` .

To recreate the original command with a double-quoted string with GNU grep :

$ { echo 'ab'; echo 'c`d'; } | grep "\\\`" # !! BOTH \ and ` need \-escaping
ab
c`d

Again, after the shell's string parsing, grep sees just \\` , which to GNU grep is the start-of-the-input anchor, so all input lines match.

• Also note that since grep processes input line by line, \\` has the same effect as ^ the start-of-a- line anchor; with multi-line input, however - such as if you used grep -z to read all lines at once - \\` only matches the very start of the whole string.

To BSD/macOS grep , \\` simply escapes a literal ` , so it only matches input lines that contain that character.