How do I change the content between tags in java?

Question

I am searching for a regex in which I can look for certain tags in FIX and then add info to them. I thought to refer to the tags as a string and search it in a regex using the ReplaceAll method. However, I don't know how do it when I don't want to replace the string in the regex but the string after it.(meaning the content in the tag).

Answer 1

Since the FIX protocol body is a set of name=value pairs separated by SOH (0x01) characters, and you want to replace the value, you can use zero-width positive lookbehind pattern.

Example: To replace value of tag 49

body = body.replaceAll("(?<=\u000149=)[^\u0001]*",
                       Matcher.quoteReplacement("new value"));

Explanation

(?<=           The matched value must be preceded by:  
  \u0001         SOH field delimiter
  49             Tag number
  =              Separating equal sign
)
[^\u0001]*     Matches value, e.g. everything up to the following SOH field delimiter

---

If you need to add text before/after/around the existing test, just insert the matched text, using the $0 reference, which also means you shouldn't use quoteReplacement() to escape literal text, but you'd have to escape any special characters yourself.

body = body.replaceAll("(?<=\u000149=)[^\u0001]*",
                       "new text before $0 new text after"));