How to extract submatches from a string and write those to a file?

Question

I want to parse a string for specific submatches and output those - one submatch per line - into a file.

Essentially I would like to extract the value of each href-attribute:

$ cat Fin
<a href="a.htm">A</a>X<a href="b.htm">B</a>Y<a href="c.htm">C</a>

$ echo Fin | [grep, sed, awk, ...?] > Fout

$ cat Fout
a.htm
b.htm
c.htm

I tried few things with named tools but things don't work or are too complicated. I would like to perform this transformation within a single command line concatenating as few as possible commands with piping.

Answer 1

You can use GNU grep (if built with PCRE support) for this:

$ grep -Po '(?<=a href=")[^"]*' file
a.htm
b.htm
c.htm

It is a look-behind (what's after a href="?) and gets everything up to next double quote.

Answer 2

sed 's%</a>%\n%g' Fin | sed 's%.*"\([a-z]\.htm\).*%\1%' > Fout
a.htm
b.htm
c.htm

Surely there is a shorter, more elegant way.