Replace .(dot) inside number Java between two elements

Following regex will match the dot character that is between numerical characters

(?<=d).(?=d)

Regex Explanation:

• d - match any digit between 0-9
• (?<=d). - positive look-behind to match any . character that has a digit just before it
• .(?=d) - positive look-ahead to match any . character that has a digit just after it

Demo:

https://regex101.com/r/WMEjPl/1

Java Code Example:

public static void main(String args[]) {
   String regex = "(?<=\d)\.(?=\d)";
  
   String str = "<p>23.5</p>";
   String str2 = "Mr. John <p>23.5</p> Hello";
   String str3 = "Mr. John <p>23.5</p> Hello 12.2324";

   System.out.println(str.replaceAll(regex, ","));   // <p>23,5</p>
   System.out.println(str2.replaceAll(regex, ","));  // Mr. John <p>23,5</p> Hello
   System.out.println(str3.replaceAll(regex, ","));  // Mr. John <p>23,5</p> Hello 12,2324
}

You may use capturing groups + escape the /:

str = str.replaceAll("(?<=<p>)(\d*)\.(\d+)(?=<\/p>)", "$1,$2");

If you want to replace dot in all numbers, you may just as well use

str = str.replaceAll("(\d*)\.(\d+)", "$1,$2");