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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