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How to replace character in the string using regex in java?

I want to replace every x in the end of line or string and behind every letters except aiueo with nya.
Expected input and output:
Input: bapakx
Output: bapaknya
I've tried this one:
String myString = "bapakx";
String regex = "[^aiueo]x(\\s|$)";
Pattern p = Pattern.compile(regex);
Matcher m = p.matcher(myString);
if(m.find()){
myString = m.replaceAll("nya");
}
But the output is not bapaknya but bapanya. The k character is also replaced. How can I solve this?

To get consonant back Use a zero width lookbehind in your regex as:
String regex = "(?<=[^aiueo])x(?=\\s|$)";
Here (?<=[^aiueo]) will only assert presence of consonant before x but won't match it.
Alternatively you can use capture groups:
String regex = "([^aiueo])x(\\s|$)";
and use it as:
myString = m.replaceAll("$1nya");

Regex to find a word between $$ sign

I want regular expression to find a word between $$ sign only. It must start and end with $ sign. I have tried below expression
final String regex = "\\$\\w+\\$";
final Pattern pattern = Pattern.compile(regex);
final Matcher matcher = pattern.matcher("$abc$ cde$efg$hij pqr");
This should give me count as 1. But my regular expression also considering second occurrence of (cde$efg$hij) which it should not consider as it is not starting and ending with $$ sign.

You may use non-word boundaries:
final String regex = "\\B\\$\\w+\\$\\B";
The pattern will only match if the $abc$ is not preceded and followed with word chars. See the regex demo.
See Java demo:
String regex = "\\B\\$\\w+\\$\\B";
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher("$abc$ cde$efg$hij pqr");
while (matcher.find()){
System.out.println(matcher.group(0));
} // => $abc$
Besides non-word boundaries, you may use whitespace boundaries if you only want to match in between whitespace chars or start/end of string:
String regex = "(?<!\\S)\\$\\w+\\$(?!\\S)";
Or, use unambiguous word boundaries (as I call them):
String regex = "(?<!\\w)\\$\\w+\\$(?!\\w)";
The (?<!\\w) negative lookbehind will fail the match if a word char is found immediately to the left of the current location, and the (?!\w) negative lookahead will fail the match if a word char is found immediately to the right of the current location.

The problem was extracting fields between dollar signs for me.
List<String> getFieldNames(#NotNull String str) {
final String regex = "\\$(\\w+)\\$";
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(str);
List<String> fields = new ArrayList<>();
while (matcher.find()) {
fields.add(matcher.group(1));
}
return fields;
}
This will return list of words between dollar signs.

JAVA split with regex doesn't work

I have the following String 46MTS007 and i have to split numbers from letters so in result i should get an array like {"46", "MTS", "007"}
String s = "46MTS007";
String[] spl = s.split("\\d+|\\D+");
But spl remains empty, what's wrong with the regex? I've tested in regex101 and it's working like expected (with global flag)

If you want to use split you can use this lookaround based regex:
(?<=\d)(?=\D)|(?<=\D)(?=\d)
RegEx Demo
Which means split the places where next position is digit and previous is non-digit OR when position is non-digit and previous position is a digit.
In Java:
String s = "46MTS007";
String[] spl = s.split("(?<=\\d)(?=\\D)|(?<=\\D)(?=\\d)");

Regex you're using will not split the string. Split() splits the string with regex you provide but regex used here matches with whole string not the delimiter. You can use Pattern Matcher to find different groups in a string.
public static void main(String[] args) {
String line = "46MTS007";
String regex = "\\D+|\\d+";
Pattern pattern = Pattern.compile(regex);
Matcher m = pattern.matcher(line);
while(m.find())
System.out.println(m.group());
}
Output:
46
MTS
007
Note: Don't forget to user m.find() after capturing each group otherwise it'll not move to next one.

Replacing Pattern Matches in a String

String output = "";
pattern = Pattern.compile(">Part\s.");
matcher = pattern.matcher(docToProcess);
while (matcher.find()) {
match = matcher.group();
}
I'm trying to use the above code to find the pattern >Part\s. inside docToProcess (Which is a string of a large xml document) and then what I want to do is replace the content that matches the pattern with <ref></ref>
Any ideas how I can make the output variable equal to docToProcess except with the replacements as indicated above?
EDIT: I need to use the matcher somehow when replacing. I can't just use replaceAll()

You can use String#replaceAll method. It takes a Regex as first parameter: -
String output = docToProcess.replaceAll(">Part\\s\\.", "<ref></ref>");
Note that, dot (.) is a special meta-character in regex, which matches everything, and not just a dot(.). So, you need to escape it, unless you really wanted to match any character after >Part\\s. And you need to add 2 backslashes to escape in Java.
If you want to use Matcher class, the you can use Matcher.appendReplacement method: -
String docToProcess = "XYZ>Part .asdf";
Pattern p = Pattern.compile(">Part\\s\\.");
Matcher m = p.matcher(docToProcess);
StringBuffer sb = new StringBuffer();
while (m.find()) {
m.appendReplacement(sb, "<ref></ref>");
}
m.appendTail(sb);
System.out.println(sb.toString());
OUTPUT : -
"XYZ<ref></ref>asdf"

This is what you need:
String docToProcess = "... your xml here ...";
Pattern pattern = Pattern.compile(">Part\\s.");
Matcher matcher = pattern.matcher(docToProcess);
StringBuffer output = new StringBuffer();
while (matcher.find()) matcher.appendReplacement(output, "<ref></ref>");
matcher.appendTail(output);
Unfortunately, you can't use the StringBuilder due to historical constraints on the Java API.

docToProcess.replaceAll(">Part\\s[.]", "<ref></ref>");

String output = docToProcess.replaceAll(">Part\\s\\.", "<ref></ref>");

RegEX: how to match string which is not surrounded

I have a String "REC/LESS FEES/CODE/AU013423".
What could be the regEx expression to match "REC" and "AU013423" (anything that is not surrounded by slashes /)
I am using /^>*/, which works and matches the string within slash's i.e. using this I am able to find "/LESS FEES/CODE/", but I want to negate this to find reverse i.e. REC and AU013423.
Need help on this. Thanks

If you know that you're only looking for alphanumeric data you can use the regex ([A-Z0-9]+)/.*/([A-Z0-9]+) If this matches you will have the two groups which contain the first & final text strings.
This code prints RECAU013423
final String s = "REC/LESS FEES/CODE/AU013423";
final Pattern regex = Pattern.compile("([A-Z0-9]+)/.*/([A-Z0-9]+)", Pattern.CASE_INSENSITIVE);
final Matcher matcher = regex.matcher(s);
if (matcher.matches()) {
System.out.println(matcher.group(1) + matcher.group(2));
}
You can tweak the regex groups as necessary to cover valid characters

Here's another option:
String s = "REC/LESS FEES/CODE/AU013423";
String[] results = s.split("/.*/");
System.out.println(Arrays.toString(results));
// [REC, AU013423]

^[^/]+|[^/]+$
matches anything that occurs before the first or after the last slash in the string (or the entire string if there is no slash present).
To iterate over all matches in a string in Java:
Pattern regex = Pattern.compile("^[^/]+|[^/]+$");
Matcher regexMatcher = regex.matcher(subjectString);
while (regexMatcher.find()) {
// matched text: regexMatcher.group()
// match start: regexMatcher.start()
// match end: regexMatcher.end()
}