What does this command do?
grep "\bi\b" linux.txt
What is it searching for?
3 Answers
\b in a regular expression means "word boundary".
With this grep command, you are searching for all words i in the file linux.txt. i can be at the beginning of a line or at the end, or between two space characters in a sentence.
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can be in the begining or in the end?? isn't supposed to be only between spaces??– amyassinOct 16, 2011 at 19:43
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1I don't find more details about that. But trying by hand with a file containing the lines "i foo" and "bar i", the regexp provided to the grep command above is matching both of them.– uloBasEIOct 16, 2011 at 19:53
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1More precisely, not only space, but any non-word character. Just like the
-w--word-regexpswitch does:grep -w "i" linux.txt. For example a line like "<i>italic</i>" also matches. Oct 17, 2011 at 6:48
The \b (word boundary) anchor can be used in place of \< and \> to signify the beginning or end of a word.
If this is the content of a file:
Hi
this
is test file
to carry out few regular expressions
Then:
$ grep -e '\breg' file to carry out few regular expressions
Note about \b , \<, \>:
When used inside quotes ("" or ''), \b and \<,\> work as word boundaries, as explained above.
When quotes are not used, \\b has to be used instead of \b.
Examples:
grep i works but does not find whole words only
grep \bi\b does not work
grep \\bi\\b works
grep "\bi\b" works and it is the same as grep "\<i\>"
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\<...\>and\b...\b: `You can get unexpected results if you assume the two patterns behave the same... see this link