Cheat sheet for Java Regex: Changing plain text, extracting substring.


Index

  1. Change plain text to Korean Phone Number format
  2. Extracting SubString

Change plain text to Korean Phone Number format

1
2
3
4
5
6
7
8

 public static void testNumber(String src) {

        Pattern SRC_PHONE_FORMAT =  Pattern.compile("(\\d{3})(\\d{4})(\\d{4})");
        Pattern DEST_PHONE_FORMAT = Pattern.compile("$1-$2-$3");

        String dest = src.replaceFirst(SRC_PHONE_FORMAT.toString(), DEST_PHONE_FORMAT.toString());
        System.out.println(dest);
 }

맨처음에는 패턴들에 통째로 replaceAll을 걸었는데 그럼 결과가 그냥 DEST_PHONE_FORMAT.toString()이랑 동일한 결과가 나온다. replaceFirst를 걸어서 ()()() 이렇게 끊어서 하나씩 적용을 시켜주면 된다. 원래 regex group으로 값들을 꺼내서 바꿀까 했는데 이 쪽이 조금 더 명시적인 느낌이 들었다.


Extracting SubString

Extracting before and after of specific character

Let’s say I get string with static prefix and I only need string after the prefix.

For example, A:COME_IN. A is prefix and I only need COME_IN. I could guess prefix is only one character, however it wouldn’t be general to many case. So I will set wild care before and after of the prefix.


Extracting before and after with :

1
2
3
4
5
6
7
8
9

String src = "A:COME_IN"; 

Pattern pattern = Pattern.compile("([^,]*):([^,]*)");
Matcher mater = pattern.matcher(src);

if(mater.find()) {
        String before = mater.group(1); // "A"
        String after = mater.group(1); // "COME_IN"
}

Code detail

  • Use find() to move forward. It works like iterator hasNext(). It keeps moving after checking a part of the src string. Finding makes matcher keep moving forward
  • group(0) return whole chosen string if src matches pattern. group(1) is ‘A’, group(2) is ‘COME_IN’

I usually don’t make matcher. I just go for pattern.matcher(src).matches because in that case I don’t have to return a string. It was just for checking existence and compatibility for format. But in this case, I have to return sub string.


Extracting between special characters

Think about string like this. I (Love) you and let’s say I want to extract Love. This will teach me how to do this.


Extract Love between ()

1
2
3
4
5
6
7

Pattern pattern = Pattern.compile("\\((.*?)\\)");
        Matcher matcher = pattern.matcher(src);

        if(matcher.find()) {
            System.out.println(matcher.group(0)); // (Love)
            System.out.println(matcher.group(1)); // Love
        }

Code detail

  • \\ is escape for ( and ). It is well known fact that some characters can cause confusion to computer, so it’s like safety lock for those kind of letters.

Leave the link about group for later. I think I’ll confused by the concept again [what is group in java regex]