Ubuntu – Extract a number in a txt file by using regular expressions

The parameter expansion's substitution doesn't work with regular expressions. extglob can help you here, but it doesn't work with regular expressions, either.

The correct syntax for the expression under extglob is

re='+([0-9])'

i.e. a digit one or more times.

Test:

re='+([0-9])'
echo $'word5word\nword55word' | while read line ; do
    new_line=${line/"word"$re"word"/"word"125"word"}
    echo "$new_line"
done

Output:

word125word
word125word

You can achieve this with just a single grep command. This is because GNU grep lets you use a Perl regular expression (-P), which supports zero-width lookaround assertions (\K and (?= ), in this case):

grep -oP '^\[\K\d+(?=\])' infile

As written, that will send the output to your terminal. To redirect it to a file, use:

grep -oP '^\[\K\d+(?=\])' infile > outfile

This method has the advantage of brevity and simplicity. It matches text that

• is preceded by (\K)

  • a [ character(\[) -- \ is needed as [ otherwise has a special meaning in regular expressions
  • that appears at the beginning of a line (^);

• consists of one or more (+) digits (\d);

• is followed by ((?= ))

  • a ] character (\]) -- like with [, \ forces ] to be matched literally.