problem of replaceAll()

Question

I have written 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?

Call me crazy, but what exactly is $1 there for? I don't see how this is relevant to whats going on. — Anthony Forloney
– Anthony Forloney, Commented Jan 27, 2010 at 2:19
I gather $1 is supposed to represent the group matched (ayyy) but Slaks has alredy said why that won't work. @Alan, try to tell us what you want to do rather than how you're trying to do it. There's almost always a better way :-) — paxdiablo
– paxdiablo, Commented Jan 27, 2010 at 2:21
@Anthony: It's supposed to match the first capture group from the regex. — SLaks
– SLaks, Commented Jan 27, 2010 at 2:22

SLaks · Accepted Answer · 2010-01-27 02:14:42Z

3

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.

answered Jan 27, 2010 at 2:14

SLaks

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3 Comments

paxdiablo Over a year ago

Good catch, @Slaks, I gather OP is trying to replace 'xxxNyyyxxx' with 'a' when N=='a' or 'b' when N!='a'. But since it's always 'a', they might as well just replace 'xxxayyyxxx' with 'a' and not fiddle around with groups.

SLaks Over a year ago

@paxdiablo: I assume that that's only an example, and that he isn't replacing a hard-coded string.

Alan Over a year ago

Thanks very much for your clear explaination! I know what to do now. Thanks

axtavt · Accepted Answer · 2010-01-27 02:16:21Z

3

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);

answered Jan 27, 2010 at 2:16

axtavt

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Comments

pgmura · Accepted Answer · 2010-01-27 02:13:00Z

-2

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)

answered Jan 27, 2010 at 2:13

pgmura

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1 Comment

SLaks Over a year ago

He realizes that; that's not his problem.

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