Check for valid link (URL)

The following works in Bash >= version 3.2 without using grep:

regex='(https?|ftp|file)://[-A-Za-z0-9\+&@#/%?=~_|!:,.;]*[-A-Za-z0-9\+&@#/%=~_|]'string='http://www.google.com/test/link.php'if [[ $string =~ $regex ]]then     echo "Link valid"else    echo "Link not valid"fi

Your regex doesn't seem to include lowercase alpha characters [a-z] so I added them here.

Since I don't have enough rep to comment above, I am going to amend the answer given by Dennis above with this one.

I incorporated Christopher's update to the regex and then added more to it so that the URL has to at least be in this format:

http://w.w (has to have a period in it).

And tweaked output a bit :)

regex='^(https?|ftp|file)://[-A-Za-z0-9\+&@#/%?=~_|!:,.;]*[-A-Za-z0-9\+&@#/%=~_|]\.[-A-Za-z0-9\+&@#/%?=~_|!:,.;]*[-A-Za-z0-9\+&@#/%=~_|]$'url='http://www.google.com/test/link.php'if [[ $url =~ $regex ]]then     echo "$url IS valid"else    echo "$url IS NOT valid"fi

Probably because the regular expression is written in PCRE syntax. See if you have (or can install) the program pcregrep on your system - it has the same syntax as grep but accepts Perl-compatible regexes - and you should be able to make that work.

Another option is to try the -P option to grep, but the man page says that's "highly experimental" so it may or may not actually work.

I will say that you should think carefully about whether it's really appropriate to be using this or any regex to validate a URL. If you want to have a correct validation, you'd probably be better off finding or writing a small script in, say, Perl, to use the URL validation facilities of the language.

EDIT: In response to your edit in the question, I didn't notice that that regex is also valid in "extended" syntax. I don't think you can get better/faster than that.