grep words around a token

Question

I have some lines in a file like:

This is one word1:word2 of the lines    
This is another word3:word4 of the lines    
Line without a match    
Yet another line word5:word6 for test

I need to grep for : and to return words just before and after :.

Output I need from greping above lines is

word1:word2
word3:word4
word5:word6

Answer 1

With GNU grep:

start cmd:> echo "This is one word1:word2 of the lines" |
  grep -Eo '[[:alnum:]]+:[[:alnum:]]+'
word1:word2

start cmd:> echo "This is one wordx:wordy of the lines" |
  grep -Eo '[[:alpha:]]*:[[:alpha:]]*'
wordx:wordy

start cmd:> echo "This is one wo_rdx:wo_rdy of the lines" |
  grep -Eo '[[:alpha:]_]*:[[:alpha:]_]*'
wo_rdx:wo_rdy

Answer 2

POSIXly (though beware some tr implementations (like GNU's) don't work properly with multi-byte characters).

tr -s '[:space:]_' '[\n*]' << 'EOF' |
  grep -xE '[[:alnum:]_]+:[[:alnum:]_]+'
This is one word1:word2 of the lines and another is word:word   
This is another word3:word4 of the lines  and this is not wordnot::wordnot
Line without a match    
Yet another line word5:word6 for test
This is one wo_rdx:wo_rdy of the lines
This is one wordx:wordy of the lines
not/a:match
EOF

Gives:

word1:word2
word:word
word3:word4
word5:word6
rdx:wo
wordx:wordy

Answer 3

For all case of your desired result, you can use GNU grep with PCRE support(-P) and its word regex (\w) as following:

grep -oP '\w+:\w+' file

Input file:

This is one word1:word2 of the lines and another is word:word   
This is another word3:word4 of the lines  and this is not wordnot::wordnot
Line without a match    
Yet another line word5:word6 for test
This is one wo_rdx:wo_rdy of the lines
This is one wordx:wordy of the lines

Output:

word1:word2
word:word
word3:word4
word5:word6
wo_rdx:wo_rdy
wordx:wordy

As you can see grep doesn't match wordnot::wordnot pattern, because it has a extra : between itself.

Answer 4

If you were to use sed to do this thing, it might be done like:

sed -n 's/\( *: *\)*\([^ ]*:[^ ]* *\)*.\{,1\}/\2/g;/./p' <<\IN 
This is one word1:word2 of the lines
This is another word3:word4 of the lines
Line without :a :match
Yet another line word5:word6 for test:123:test
IN

OUTPUT

word1:word2 
word3:word4 
word5:word6 test:123:test

Answer 5

Through grep,

grep -oP '[^:\s]+:[^:\s]+' file

OR

grep -oP '\S+?:\S+' file

The above command not only fetches the strings like foo:bar but also ?foo:bar?