I want to prompt the user to input a URL, but it can only contain A-Z, a-z, 0-9, &, ., /, =, _, -, :, and ?.

So, for example:

Enter URL:

$ http://youtube.com/watch?v=1234df_AQ-x

That URL is allowed.



Enter URL:

$ https://unix.stackexchange.com/$FAKEurl%🍺123

That URL is NOT allowed.

This is what I've come up with so far, but it doesn't seem to work correctly:

if [[ ! "${URL}" == *[abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890-_/&?:.=]* ]]; then

    echo "That URL is NOT allowed."

else

    echo "That URL is allowed."

fi

Please note that the URLs I provided in the example are just examples. 
This script needs to work with all possible user input; it just can't contain characters other than the ones I specified earlier.

Using bash 3.2.57(1)-release under macOS High Sierra 10.13.6.

Another way is with a plain case statement:

#!/bin/sh



case $URL in

  (*[^A-Za-z0-9&./=_:?-]*) echo not allowed;;

  (*) echo allowed;;

esac

The basic idea is to invert the logic and ask if there are any characters in the argument that are not in the allowed range.

Your attempt

Your attempt might not work as expected because it considers all URLs as allowed that contain at least one of the allowed characters. Formally, you compare the URL with

<anything><allowed_character><anything>

If this does not match, you reject the URL.

This might help

If you replace your if ... else ... fi by

if [[ "${URL}" =~ [^A-Za-z0-9&./=_-:?] ]]; then

    echo "That URL is NOT allowed."

else

    echo "That URL is allowed."

fi

it might do what you want.

Here, the binary operator =~ is used to find a match of the regular expression [^A-Za-z0-9&./=_-:?] within "${URL}". This operator does not require that the whole string "${URL}" matches the regular expression, any matching substring will do. Such a match is found for any character that is not allowed in the URL. The "not" comes from the leading caret (^) in the definition of the character set. Please note that there is no negating ! in the conditional expression any more.

If "${URL}" contains a forbidden character, the regular expression matches and the compound command [[...]] evaluates to true (zero exit status).

Your current logic is wrong, it returns true only when every character in the input URL exists outside the set of permitted characters.

Try something along the lines of

if [[ "${URL}"  = *[!A-Za-z0-9&./=_:?-]* ]]

then

  echo "That URL is NOT allowed."

else

  echo "That URL is allowed"

fi

This check, due to the negation (!) inside the character range, is intended to return true when the input URL contains one or more characters outside of the permitted set