Question

Given String

//          1       2       3
String a = "letters.1223434.more_letters";

I’d like to recognize that numbers come in a 2nd position after the first dot

I then would like to use this knowledge to replace “2nd position of”

//          1         2           3
String b = "someWords.otherwords.morewords";

with “hello” to effectively make

//          1         2     3
String b = "someWords.hello.morewords";

Substitution would have to be done based on the original position of matched element in String a

How can this be done using regex please?

Answer 1

For finding those numbers you can use group mechanism (round brackets in regular expresions):

import java.util.regex.*;

...

String data = "letters.1223434.more_letters";
String pattern="(.+?)\\.(.+?)\\.(.+)";
Matcher m = Pattern.compile(pattern).matcher(data);
if (m.find()) //or while if needed
    for (int i = 1; i <= m.groupCount(); i++) 
    //group 0 == whole String, so I ignore it and start from i=1
        System.out.println(i+") [" + m.group(i) + "] start="+m.start(i));
// OUT:
//1) [letters] start=0
//2) [1223434] start=8
//3) [more_letters] start=16

BUT if your goal is just replacing text between two dots try maybe replaceFirst(String regex, String replacement) method on String object:

//find ALL characters between 2 dots once and replace them 
String a = "letters.1223434abc.more_letters";
a=a.replaceFirst("\\.(.+)\\.", ".hello.");
System.out.println(a);// OUT => letters.hello.more_letters

regex tells to search all characters between two dots (including these dots), so replacement should be “.hello.” (with dots).

If your String will have more dots it will replace ALL characters between first and last dot. If you want regex to search for minimum number of characters necessary to satisfy the pattern you need to use Reluctant Quantifier ->? like:

String b = "letters.1223434abc.more_letters.another.dots";
b=b.replaceFirst("\\.(.+?)\\.", ".hello.");//there is "+?" instead of "+"
System.out.println(b);// OUT => letters.hello.more_letters.another.dots