problem of replaceAll()

I have wr开发者_如何学编程itten the following line of code:

str.replaceAll("xxx(ayyy)xxx", "$1".substring(0,1).equals("a") ? "a" : "b");

But I found that "$1".substring(0,1) will output "$" instead of "a". Is that any way to solve this problem?

The second parameter to replaceAll is a regular string.

Java will evaluate your parameter before passing it to the function, not for each match.
"$1".substring(0,1) simply returns the first character in the string $1.

You need to call the appendReplacement method of the Matcher class in a loop.

If you want to apply different replacements for each match, use appendReplacement/appendTail:

Pattern p = Pattern.compile("xxx(ayyy)xxx");
StringBuffer out = new StringBuffer();
Matcher m = p.matcher("...");
while (m.find()) {
    m.appendReplacement(out, m.group(1).substring(0, 1).equals("a") ? "a" : "b");
}
m.appendTail(out);

substring(start, end) will get you a substring from start till one before end. If you want to eliminate the first element try substring(1,lengthOfString)