Question

Can anyone advise? And how does the replace all method works?

message = message.replaceAll("(?:https?|http?)://[\\w/%.\\-?&=!#]+",
"<a href='$0' target='_blank'>$0</a>");

Answer 1

You should be taken this each step at a time to understand:

(?:https?|http?)

| -> means OR so it means matches either https or http

? -> (only here) means optional elements, “s” from https is optional or “p” from http is optional too. What you probably meant is to do : “(https)?|(http)?“, make each optional. But this is still wrong, because you then could match a String like this one: “://someLink” – which does not make sense.

I suppose you want to match https OR http, thus your regex here needs to changed to :

 https? (to make only the s optinal)

?: -> it defines a non-capturing group (it means it will be matched but not present in the output – not captured in the result), this is something related to regex groups.

() -> mean that this is a group
and as matter of fact this is the second group (the first is the whole match itself)

This regex “://[\\w/%.\\-?&=!#]+” has been explained above pretty good I think.

Here is the “magic” that happens here: “$0“, this is called a backreference to a matching group. So inside your message String, the result that you have matched with the regex:

(?:https?|http?)://[\\w/%.\\-?&=!#]+

will be replaced with this:

"<a href='$0' target='_blank'>$0</a>"

where $0 is actually the thing that was found with the first regex.

Here is an example actually:

// I've simplified: \\w+ any number of characters
String regex = "https?://\\w+";
String input = "http://value1 ; https://value2";
input = input.replaceAll(regex, "<a href='$0' target='_blank'>$0</a> ");

// this will output : 
// <a href='http://value1' target='_blank'>http://value1</a>  ; <a href='https://value2' target='_blank'>https://value2</a>
System.out.println(input);