Why does String.replaceAll(regex) replace twice when using \'.*\'?

Why String.replaceAll(regex) Replaces Twice

In the code snippet:

System.out.println("test".replaceAll(".*", "a"));

the regular expression .* matches any character, including zero characters. This means that it can match the entire string twice:

• During the first pass, .* matches the entire string and replaces it with "a."
• During the second pass, .* matches an empty string at the end of the input (since it can match zero characters) and replaces it with another "a."

This behavior is not considered a bug in the Java regex engine. Instead, it is a consequence of the way .* matches any character.

Alternatives

To avoid this behavior, you can use the following alternatives:

• Use .replaceFirst() to only replace the first occurrence:

"test".replaceFirst(".*", "a")

• Use .matches() to check if the entire string matches the given regular expression:

System.out.println("test".matches(".*")); // Prints true

• Use a more specific regular expression, such as . which requires at least one character:

System.out.println("test".replaceAll(".+", "a")); // Prints a

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