You can try the regular expression:
[a-z]+n't\s*
e.g.
private static final Pattern REGEX_PATTERN =
Pattern.compile("[a-z]+n't\\s*",
Pattern.CASE_INSENSITIVE | Pattern.MULTILINE);
public static void main(String[] args) {
String input = "doesn't like\nI don't like\nI don't like having dinner now";
System.out.println(
REGEX_PATTERN.matcher(input).replaceAll("not_")
);
}
Output:
not_like
I not_like
I not_like having dinner now