Java regex match anything except a single expression

Question

I am trying to replace everything except a specific expression including digits in java using only the replaceAll() method and a single regex.

Given the String P=32 N=5 M=2 I want to extract each variable independently. I can match the expression N=5 with the regex N=\d, but I can't seem to find an inverse expression that will match anything but N=\d, where x may be any digit.

I do not want to use Pattern or Matcher but solve this using regex only. So for x, y, z being any digit, I want to be able to replace everything but the expression N=y in a String P=x N=y M=z:

String input = "P=32 N=5 M=2";
output = input.replaceAll(regex, "");
System.out.println(output);
// expected "N=5"

Possible duplicate of [RegEx to exclude a specific string constant](https://stackoverflow.com/questions/1395177/regex-to-exclude-a-specific-string-constant) — www.hybriscx.com, Nov 14 '19 at 13:46
@YCF_L after using `replaceAll(regex, "")` only my String expression, e.g. `N=5` — T A, Nov 14 '19 at 13:53
@WiktorStribiżew this will work for the specific case that the String is N=5; I want to match any String N=x where X is any digit — T A, Nov 14 '19 at 14:03
@WiktorStribiżew I want to match not only N=5 but also possibly e.g. N=1 or N= 12 — T A, Nov 14 '19 at 14:06
Yeah, replace the `5\b` with `\d+\b` or just keep `N=` in the lookahead. It depends on whether your input is messy or not. — Wiktor Stribiżew, Nov 14 '19 at 14:11

score 1 · Accepted Answer · answered Nov 14 '19 at 14:07

You may use

s = s.replaceAll("\\s*\\b(?!N=\\d)\\w+=\\d+", "").trim();

See the Java demo and the regex demo.

Details

\s* - 0+ whitespaces
\b - a word boundary
(?!N=\d) - immediately to the right, there should be no N= and any digit
\w+ - 1+ letters/digits/_
= - an = sign
\d+ - 1+ digits.

Java regex match anything except a single expression

1 Answers1