• Uncategorized

About grep : Can-grep-show-only-words-that-match-search-pattern

Question Detail

Is there a way to make grep output “words” from files that match the search expression?

If I want to find all the instances of, say, “th” in a number of files, I can do:

grep "th" *

but the output will be something like (bold is by me);

some-text-file : the cat sat on the mat  
some-other-text-file : the quick brown fox  
yet-another-text-file : i hope this explains it thoroughly 

What I want it to output, using the same search, is:

the
the
the
this
thoroughly

Is this possible using grep? Or using another combination of tools?

Question Answer

Try grep -o:

grep -oh "\w*th\w*" *

Edit: matching from Phil’s comment.

From the docs:

-h, --no-filename
    Suppress the prefixing of file names on output. This is the default
    when there is only  one  file  (or only standard input) to search.
-o, --only-matching
    Print  only  the matched (non-empty) parts of a matching line,
    with each such part on a separate output line.

Cross distribution safe answer (including windows minGW?)

grep -h "[[:alpha:]]*th[[:alpha:]]*" 'filename' | tr ' ' '\n' | grep -h "[[:alpha:]]*th[[:alpha:]]*"

If you’re using older versions of grep (like 2.4.2) which do not include the -o option, then use the above. Else use the simpler to maintain version below.

Linux cross distribution safe answer

grep -oh "[[:alpha:]]*th[[:alpha:]]*" 'filename'

To summarize: -oh outputs the regular expression matches to the file content (and not its filename), just like how you would expect a regular expression to work in vim/etc… What word or regular expression you would be searching for then, is up to you! As long as you remain with POSIX and not perl syntax (refer below)

More from the manual for grep

-o      Print each match, but only the match, not the entire line.
-h      Never print filename headers (i.e. filenames) with output lines.
-w      The expression is searched for as a word (as if surrounded by
         `[[:<:]]' and `[[:>:]]';

The reason why the original answer does not work for everyone

The usage of \w varies from platform to platform, as it’s an extended “perl” syntax. As such, those grep installations that are limited to work with POSIX character classes use [[:alpha:]] and not its perl equivalent of \w. See the Wikipedia page on regular expression for more

Ultimately, the POSIX answer above will be a lot more reliable regardless of platform (being the original) for grep

As for support of grep without -o option, the first grep outputs the relevant lines, the tr splits the spaces to new lines, the final grep filters only for the respective lines.

(PS: I know most platforms by now would have been patched for \w…. but there are always those that lag behind)

Credit for the “-o” workaround from @AdamRosenfield answer

It’s more simple than you think. Try this:

egrep -wo 'th.[a-z]*' filename.txt #### (Case Sensitive)

egrep -iwo 'th.[a-z]*' filename.txt  ### (Case Insensitive)

Where,

 egrep: Grep will work with extended regular expression.
 w    : Matches only word/words instead of substring.
 o    : Display only matched pattern instead of whole line.
 i    : If u want to ignore case sensitivity.

You could translate spaces to newlines and then grep, e.g.:

cat * | tr ' ' '\n' | grep th

Just awk, no need combination of tools.

# awk '{for(i=1;i<=NF;i++){if($i~/^th/){print $i}}}' file
the
the
the
this
thoroughly

grep command for only matching and perl

grep -o -P 'th.*? ' filename

I was unsatisfied with awk’s hard to remember syntax but I liked the idea of using one utility to do this.

It seems like ack (or ack-grep if you use Ubuntu) can do this easily:

# ack-grep -ho "\bth.*?\b" *

the
the
the
this
thoroughly

If you omit the -h flag you get:

# ack-grep -o "\bth.*?\b" *

some-other-text-file
1:the

some-text-file
1:the
the

yet-another-text-file
1:this
thoroughly

As a bonus, you can use the --output flag to do this for more complex searches with just about the easiest syntax I’ve found:

# echo "bug: 1, id: 5, time: 12/27/2010" > test-file
# ack-grep -ho "bug: (\d*), id: (\d*), time: (.*)" --output '$1, $2, $3' test-file

1, 5, 12/27/2010

cat *-text-file | grep -Eio "th[a-z]+"

You can also try pcregrep. There is also a -w option in grep, but in some cases it doesn’t work as expected.

From Wikipedia:

cat fruitlist.txt
apple
apples
pineapple
apple-
apple-fruit
fruit-apple

grep -w apple fruitlist.txt
apple
apple-
apple-fruit
fruit-apple

To search all the words with start with “icon-” the following command works perfect. I am using Ack here which is similar to grep but with better options and nice formatting.

ack -oh --type=html "\w*icon-\w*" | sort | uniq

I had a similar problem, looking for grep/pattern regex and the “matched pattern found” as output.

At the end I used egrep (same regex on grep -e or -G didn’t give me the same result of egrep) with the option -o

so, I think that could be something similar to (I’m NOT a regex Master) :

egrep -o "the*|this{1}|thoroughly{1}" filename

You could pipe your grep output into Perl like this:

grep "th" * | perl -n -e'while(/(\w*th\w*)/g) {print "$1\n"}'

$ grep -w

Excerpt from grep man page:

-w: Select only those lines containing matches that form whole words. The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character.

ripgrep

Here are the example using ripgrep:

rg -o "(\w+)?th(\w+)?"

It’ll match all words matching th.

You may also like...

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.