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When I search for tabs in a file with (e)grep I use the litteral tab (^v + <tab>). I can not utilize \t as a replacement for tabs in regular expressions. With e.g. sed this expression works very well.

So is there any possibility to use a non-litteral replacement for <tab> and what are the backgrounds for a non working / not interpreted \t ?

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up vote 99 down vote accepted

grep is using regular expressions as defined by POSIX. For whatever reasons POSIX have not defined \t as tab.

You have several alternatives:

  • tell grep to use the regular expressions as defined by perl (perl has \t as tab):

    grep -P "\t" foo.txt
  • use printf to print a tab character for you:

    grep "$(printf '\t')" foo.txt
  • or, as you have already mentioned, use the literal tab character:

    grep "^V<tab>" foo.txt

    that is: type grep ", then press ctrl+v, then press tab, then type " foo.txt. pressing ctrl+v in the shell causes the next key to be taken literally. that means the shell will insert a tab character instead of triggering some function bound to the tab key.

  • if you use bash you can use the ansi c quoting feature:

    grep $'\t' foo.txt

See the wikipedia article about regular expressions for an overview of the defined character classes in POSIX and other systems.

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Thank you for your exact explanations. – Lasall Jul 14 '11 at 15:13
basing on enzotib's answer let me add the following: grep $'\t' foo.txt (but I would usually write fgrep instead of grep) – Walter Tross Feb 21 '13 at 10:16
I needed this, combined with using the value of an environment variable. I used grep "$(printf '\t')${myvar}" foo.txt. It worked fine. With a few tries, I could not get the last form to work. – sancho.s Nov 23 '15 at 22:03
Is there any reason that plain grep couldn't silently interpret \t as tab? Does POSIX require that \t mean something else? Perhaps it's supposed to match only a literal \ followed by a t? – Aaron McDaid Jan 11 at 14:23

It is not exactly the answer you would to ear, but a possible use of escape sequences is provided by bash

command | grep $'\t'

(do not put it into double quotes!).

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Thank you for publish your suggestion. Even it does not make use of greps own mechanisms it is a smart solution. – Lasall Jul 14 '11 at 15:13
there is no need for the -E (what is searched for is no regex). There is also no need to pipe from a command. That said, thank you for pointing out this quite overlooked feature of bash (single-quoted strings preceded by $) – Walter Tross Feb 21 '13 at 10:29
Indeed, I suggest that @enzotib edit the answer to be simply grep $'\t'. – Teemu Leisti Jun 4 '13 at 12:01

regex defines that [[:space:]] searches for both spaces and tabs. u can also try:

egrep "[:blank:]{2,5}

this will find most of the tabs (since mostly a tab is 2-5 spaces long.

using \t in sed is probably an enhancement of sed itselves.

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A tab isn't any number of spaces long, it is an entirely separate character. It just looks like a number of spaces when it is shown by a text editor. – TooManyKooks Jul 14 '11 at 14:45
Thank you for taking the time to answer this question. But the character class [:blank:] includes spaces and tabs. So your solution unfortunately does not distinguish between them. – Lasall Jul 14 '11 at 15:11

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