Java replace initial characters in String and preserve length

user_2326893 0
Viewed 384

I'm looking for solution (Java) to replace the initial occurrences of multiple character with the same number of other character, for example if 'a' should be replaced with '-', than I expect:

aaabbaa  -> ---bbaa
aaxxaab  -> --xxaab
xaaaaax  -> xaaaaax

I've tried something like:

"aaabbaa".replaceAll( "^[a]+", "-")        // -bbaa
"aaabbaa".replaceAll( "(?=^[a]+)", "-")    // -aaabbaa

If possible, I prefer regex or oneliner.

Do you have any hints?

regards, Annie

regexp-replace regex java
3 Answers

If using Java 9+, use the lambda overload for replaceAll:

import java.util.regex.Pattern;

class Main {
    public static void main(String[] args) {
        String res = Pattern
            .compile("^a+")
            .matcher("aaabbb")
            .replaceAll(m -> "-".repeat(m.group().length()));
        System.out.println(res); // => ---bbb
    }
}
user_62433520

Java supports finite repetition in a lookbehind. You could match a asserting what is on the left from the start of the string are only a's.

In the replacement using -

(?<=^a{1,100})a

Regex demo | Java demo

For example

System.out.println("aaabbaa".replaceAll("(?<=^a{0,100})a", "-"));

Output

---bbaa
user_54249880

Use a char[]:

char[] cs = str.toCharArray();
for (int i = 0; i < cs.length && cs[i] == 'a'; ++i) {
  cs[i] = '-';
}
String newStr = new String(cs);
user_37881760
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