How to print only a first match from each line?

Question

I have a file that looks like this:

asd 123 aaa wrqiqirw 123
123 itiewth 123 asno 123
123 132 123 123 123
boagii 123 asdnojaneoienton 123

Expected output is:

123
123
123
123

I will need to search for patterns via regex. Is there any way to implement such a thing?

Answer 1

With pcregrep, with a pattern like 12*3:

pcregrep -o1 '(12*3).*'

With pcregrep or GNU grep -P:

grep -Po '^.*?\K12*3'

(pcregrep works with bytes more than characters, while GNU grep will work on characters as defined in the current locale (and you'd have to make sure the input contains valid text in the current locale)).

Note that GNU grep won't print anything if the pattern matches the empty string.

Answer 2

In Perl, simply

perl -lne 'print $& if /\d+/' inputfile

or from stdin:

echo foo 123 bar 456 doo 789 | perl -lne 'print $& if /\d+/'
123

The regex \d+ will match any string of consecutive numbers, and $& refers the matching string.

Answer 3

Just a grep should be enough to bring matches of 123 in every line.
It does not makes sense if the match is first ,middle or in the end.
You ask for 123 you get 123 if it is in the line (unless your question is not expressed correctly and you require something different)

$ grep -wo '123' file # -w: word match  -o : return only matched string instead of the whole line (default grep operation)

In case you need to catch with regex the first number of each row (any number - any length) then this will do the job:

cat <<EOF >file1
asd 111 777 aaa wrqiqirw 123
333 123 itiewth 123 asno 123
4444 111 123 123 567
boagii what 666 asdnojaneoienton 123
EOF
grep -Po '^[0-9]+|^.*?\K[0-9]+' file1
#output
111
333
4444
666

Answer 4

POSIXLY:

LC_ALL=C sed -e 's/.*\(123\).*/\1/' <file

LC_ALL-C is needed here to prevent sed from crashing or producing unexpected result if the file contained invalid characters in your current locale.

It also produces one entry at a line, but matched the last, not the first.

For matching the first, with ast-open's sed whose EREs support the Perl-style *? non-greedy repetition operator:

LC_ALL=C sed -E 's/.*?(123).*/\1/'

(-E for extended RE will be in next version of POSIX)

Answer 5

With grep in every line:

while IFS= read -r line; do printf '%s\n' "$line" | grep -o 123 | head -1; done < filename

That is:

• While loop in order to check each line separately.
• grep -o to get only the match instead of the whole line with matches.
• head -1 to take only the first match and not the following ones.

Answer 6

sed -e '
   /\n/{P;d;}
   s/12*3/\n&\n/;D
' < inoutfile

Answer 7

with awk

re='12*3' awk '{match($0, ENVIRON["re"])}; RSTART{print(substr($0, RSTART, RLENGTH))}' file