String replace a url with part of the same url in javascript

Question

I have string that contains a random url:

http://google.com/vocab/prefix#Billy

That needs to be transformed so that everything up to, and including the first # is replaced with the value between the last / and the first # followed by a : .

The result would be:

prefix:Billy

More examples:

http://some.url/a/path#elephant --> path:elephant
http://random.com/cool/black/beans/thing#Bob --> thing:bob

I understand how to capture the prefix part /([^\\/]+(?=#))/ , but I'm struggling to do a string replace because I can't figure out how to capture the part I need to replace.

myString.replace(/([^\/]+(?=#))/, '$1:')

I would prefer to use string.replace with regex if at all possible

Answer 1

When using replace method you need to match all the patterns you want to replace instead of just the part you need to keep; Here are two options:

 let s = 'http://google.com/vocab/prefix#Billy' // using greedy regex console.log(s.replace(/.*\\/([^#]+)#/, '$1:')) // adapted from OP's attempt console.log(s.replace(/.*?([^\\/]+?)#/, '$1:')) 

Note .* part to match the substring you want to discard, () to capture the pattern you want to keep, then reformat the output.

Answer 2

Try this code:

var myString = "http://google.com/vocab/prefix#Billy";
var hashIndex = myString.indexOf("#"); // find where the "#" is

for(var i = hashIndex; i > 0; i--) { // loop from "#" index *back* to closest "/" symbol
  if(myString[i] == "/") break; // break loop when "/" is found
}

myString = myString.replace("#", ":"); // replace "#" with ":" as in your example

console.log(myString.substring(i, hashIndex); // output 

Shortened:

var myString = "http://google.com/vocab/prefix#Billy".replace("#",":");
for(var i = myString.indexOf(":"); i > 0; i--) { if(myString[i] == "/") break; }
console.log(myString.substring(i, myString.indexOf(":");