sed: extracting value of a key-value pair in a URL query string

Even simpler, if you just want the abc:

 echo 'http://www.youtube.com/watch?v=abc&g=xyz' | awk -F'[=&]' '{print $2}'

If you want the xyz :

echo 'http://www.youtube.com/watch?v=abc&g=xyz' | awk -F'[=&]' '{print $4}'

EXPLANATION:

• awk : is a scripting language that automatically processes input files line by line, splitting each line into fields. So, when you process a file with awk, for each line, the first field is $1, the second $2 etc up to $N. By default awk uses blanks as the field separator.

• -F'[=&]' : -F is used to change the field delimiter from spaces to something else. In this case, I am giving it a class of characters. Square brackets ([ ]) are used by many languages to denote groups of characters. So, specifically, -F'[=&]' means that awk should use both & and = as field delimiters.

• Therefore, given the input string from your question, using & and = as delimiters, awk will read the following fields:

  http://www.youtube.com/watch?v=abc&g=xyz
  |----------- $1 -------------| --- - ---      
                                  |  |  |
                                  |  |  ̣----- $4
                                  |  -------- $3
                                  ----------- $2
  

  So, all you need to do is print whichever one you want {print $4}.

You said you also want to check that the string is a valid youtube URL, you can't do that with sed since if it does not match the regex you give it, it will simply print the entire line. You can use a tool like Perl to only print if the regex matches:

echo 'http://www.youtube.com/watch?v=abc&g=xyz' | 
  perl -ne 's/http.*www.youtube.com\/watch\?v=(.+?)&.+/$1/ && print'

Finally, to simply print abc you can use the standard UNIX tool cut:

echo 'http://www.youtube.com/watch?v=abc&g=xyz' | 
  cut -d '=' -f 2 | cut -d '&' -f 1

if you need "xyz" try this (GNU sed):

echo 'http://www.youtube.com/watch?v=abc&g=xyz' | sed 's/.*=\([[:alnum:]]*\).*/\1/'