Negate a character group to replace all other characters

Issue

I have the following string:

"Thu Dec 31 22:00:00 UYST 2009"

I want to replace everything except for the hours and minutes so I get the following result:

"22:00"

I am using this regex :

(^([0-9][0-9]:[0-9][0-9]))

But its not matching anything.

This would be my line of actual code :

println("Thu Dec 31 22:00:00 UYST 2009".replace("(^([0-9][0-9]:[0-9][0-9]))".toRegex(),""))

Can someone help me to correct the regex?

Solution

The reason the one you have isn't working is because you are asserting that the line starts right before the minutes and seconds, which isn't the case. This can be fixed by removing the assertion (^).

If you need the assertion to remain, there is another way. In most languages, you wouldn't be able to use a variable-length positive lookbehind here, but lucky for you, it looks like you can in Kotlin.

A positive lookbehind is basically just telling the pattern "this comes before what I'm looking for". It's denoted by a group beginning with ?<=. In this case, you can use something like (?<=^[\w ]+). This will match all word characters or spaces between the beginning of the line and where the pattern that comes after it is able to match. Appending it to your expression would look something like (?<=^[\w ]+)([0-9][0-9]:[0-9][0-9]) (note you will have to escape the \w in order for it to be in a string and not be angry about it).

Side note, Yogesh_D is correct in saying that \d\d:\d\d is the same as your [0-9][0-9]:[0-9][0-9]. Using this, it would look more like (?<=^[\w ]+)\d\d:\d\d.

Answered By - Jesse