Question

I’m trying to understand the following code:

Pattern.compile("(.*?):")

I already did some research about what it could mean,
but I don’t quite get it:

According to the java docs the * would mean 0 or more times,
while ? means once or not at all.

Also, what does the ‘:’ mean?

Thanks

Answer 1

The ? after greedy operators such as + or * will make the operator non greedy. Without the ?, that regex will keep matching all the characters it finds, including the :.

As it is, the regex will match any string which happens before the semi colon (:). In this case, the semicolon is not a special character. What ever comes before the semicolon, will be thrown into a group, which can be accessed later through a Matcher object.

This code snippet will hopefully make things more clear:

    String str = "Hello: This is a Test:";
    Pattern p1 = Pattern.compile("(.*?):");
    Pattern p2 = Pattern.compile("(.*):");
    
    Matcher m1 = p1.matcher(str);
    if (m1.find())
    {
        System.out.println(m1.group(1));            
    }
    
    Matcher m2 = p2.matcher(str);
    if (m2.find())
    {
        System.out.println(m2.group(1));
    }

Yields:

Hello

Hello: This is a Test