Using sed to grep for variable and copy this with an insert text on the same line

Question

I want to grep all title="<text>" variables e.g. title="best-pencil" and copy it on the same line as alt="best-pencil" within a HTML file on Linux:

<p class="images"><img src="my-favorite-pencil.jpg" title="best-pencil">

To:

<p class="images"><img src="my-favorite-pencil.jpg" title="best-pencil" alt="best-pencil">

How can this be done using sed?

Answer 1

If you can be sure that the string will i) will always be in double quotes and will never contain " and ii) always be on just one line, then it's trivial:

$ sed -E 's/title=("[^"]*")/& alt=\1/' file
<p class="images"><img src="my-favorite-pencil.jpg" title="best-pencil" alt="best-pencil">

The trick is to look for title=" followed by 0 or more non-" until the next " and "capture" this (that's what the parentheses around ("[^"]*") do) so we can then refer to it as \1. In sed, the & symbol means "whatever was matched". So, here, we are replacing title="foo" with itself, followed by alt="foo".

If you have a sed version that doesn't support -E, you can use this instead:

$ sed 's/title=\("[^"]*"\)/& alt=\1/' file
<p class="images"><img src="my-favorite-pencil.jpg" title="best-pencil" alt="best-pencil">