How do I extract text fragments of a file using sed?

Question

I'm trying to create a BASH script to extract fragments of text from a file that has many lines similar to this one:

"11","category/subcategory/sub-subcategory/item-1","index.php?option=com_trombinoscopeextended&Itemid=125&lang=es&view=trombinoscope","251","0","0000-00-00","","","","","","","0"

From each line I only have to keep the:

category/subcategory/sub-subcategory/item-1

The context can be deleted. I was trying using sed but I can't find a way to do that. I don't understand how create a command using the regular expression that I already have:

\w+(\/[\w-]+)+

I have been reading and already tried this command and other similar ones but I don't know much about this:

cat file.txt | sed -i -E "s/\w+(\/[\w-]+)+"

That definitely is not working and I can't find a beginners guide on how to use sed and regular expressions.

Answer 1

There's nothing wrong with your regex per se, just that you fell into the trap for the unwary. Inside of the [] the \w are two literals a backslash and a "w" and NOT a "\w" as one might think it to be or as in Perl, from where it's borrowed but not fully.

One way would be to lay out explicitly what is in a \w and then proceed:

sed -Ee '
    /\n/{P;D;}
    s|\w+(/[-_A-Za-z0-9]+)+|\n&\n|;D
'

Answer 2

Parsing a file like this is better done with awk:

awk -F, '{ print $2 }' file

or cut,

cut -d, -f 2 file

This would give you "category/subcategory/sub-subcategory/item-1" (including the quotes) from the data that you supplied.

With sed, and utilizing the regular expression that you provided (but with \w replaced by [[:alnum:]]):

sed -E 's@.*,"([[:alnum:]]+(/[[:alnum:]-]+)+)",.*@\1@' file

This replaces the whole line with the bit that matches the regular expression within the first set of parentheses. This returns category/subcategory/sub-subcategory/item-1 for the data that you provided.

For more general CSV file parsing, including correctly handling quoting and fields that may contain commas or double quotes, use csvkit:

csvcut -c 2 file