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Putnik
Asked: 2012-01-20 06:25:46 +0800 CST

grep and tabs: no match

  • 772
#cat sample
1   -1  2
2   2   2
2   1-1 3

I need to get all the lines that contains negative value, that is 1st line only. Values are tab-separated. First I tried 

# grep "\-1" sample
1   -1  2
2   1-1 3

But if I try 

 grep "\t\-1" sample

I get nothing (no match). What am I doing wrong?

linux bash grep

4 Answers

  1. Best Answer

    In Bash, you can do this:

    grep $'\t-1' sample

    The $'' causes escapes to be interpreted.

    It's not necessary to escape the minus sign.

    • 3

  2. Try something like:

    grep -P '\t-1' sample

    By the way, the sample you've provided is tab-indented. You may try the following for generic whitespace matching:

    grep -P '\s-1' sample
    • 1

  3. There is no facility for \t being tab in basic regular expressions (those described by man 7 regex). As Georgi Hristozov pointed out, you can use Perl Compatible Regular Expressions to get this ability. Some implementations of grep won't support -P, but other basic tools will have more rich regular expression languages, here are some examples which should work:

    sed -n '/\t-/p' sample
awk '/\t-/' sample
perl -ne '/\t-/ && print' sample

    In order to get it working with regular grep (without using pcre) you are going to have to put a literal tab expression in your regex. In many shells where the tab key does something like completion, you can get a literal tab character with Ctrl-v<tab>. so you'd type:

    grep 'Ctrl-v<tab>-' sample
    • 1

  4. Check man 7 regex for the POSIX regular expressions you can use, while -P for PCRE is supported in lots of places its not available everywhere (and the manualpage says its buggy so I don't always trust it myself) the POSIX ones should work in more places.

    You can try something like this:

    grep "^[[:digit:]]\+[[:space:]]\+-"

    ^ matches the beginning of the line.
    [[:digit:]]\+ matches one or more digits.
    [[:space:]]\+ matches one or more whitespace characters.
    - finally matches the negative value you were looking for.

    Manual page for regex(7): http://linux.die.net/man/7/regex

    Optional solution:

    You can use awk to do this too (and certainly other tools). This is an awk example that prints lines where the second column is less than 0 (negative):

    awk '{if ($2 < 0) print}'
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