Can't find leak in my code - cocoa

Question

I've been spending the last few hours trying to find the memory leak in my code. Here it is:
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
expression = [expression stringByTrimmingCharactersInSet:
[NSCharacterSet whitespaceAndNewlineCharacterSet]]; // expression is an NSString object.
NSArray *arguments = [NSArray arrayWithObjects:expression, [#"~/Desktop/file.txt" stringByExpandingTildeInPath], #"-n", #"--line-number", nil];
NSPipe *outPipe = [[NSPipe alloc] init];
NSTask *task = [[NSTask alloc] init];
[task setLaunchPath:#"/usr/bin/grep"];
[task setArguments:arguments];
[task setStandardOutput:outPipe];
[outPipe release];
[task launch];
NSData *data = [[outPipe fileHandleForReading] readDataToEndOfFile];
[task waitUntilExit];
[task release];
NSString *string = [[NSString alloc] initWithBytes:[data bytes] length:[data length] encoding:NSUTF8StringEncoding];
string = [string stringByReplacingOccurrencesOfString:#"\r" withString:#""];
int linesNum = 0;
NSMutableArray *possibleMatches = [[NSMutableArray alloc] init];
if ([string length] > 0) {
NSArray *lines = [string componentsSeparatedByString:#"\n"];
linesNum = [lines count];
for (int i = 0; i < [lines count]; i++) {
NSString *currentLine = [lines objectAtIndex:i];
NSArray *values = [currentLine componentsSeparatedByString:#"\t"];
if ([values count] == 20)
[possibleMatches addObject:currentLine];
}
}
[string release];
[pool release];
return [possibleMatches autorelease];
I tried to follow the few basic rules of Cocoa memory management, but somehow there still seems to be a leak, I believe it's an array that's leaking. It's noticeable if possibleMatches is large. You can try the code by using any large file as "~/Desktop/file.txt" and as expression something that yields many results when grep-ing.
What's the mistake I'm making?
Thanks for any help!
-- Ry
EDIT: I just used the Clang Static Analyzer to find leaks in my code, but it doesn't find any. It only finds dead initializations, but those can't be responsible for my leaks...

Answer 1

Here:
NSString *string = [[NSString alloc] initWithBytes:[data bytes] length:[data length] encoding:NSUTF8StringEncoding];
string = [string stringByReplacingOccurrencesOfString:#"\r" withString:#""];
You're overwriting the string object pointer without releasing or autoreleasing the original string. Instead of releasing string at the end of the method, do:
NSString *string = [[[NSString alloc] initWithBytes:[data bytes] length:[data length] encoding:NSUTF8StringEncoding] autorelease];
string = [string stringByReplacingOccurrencesOfString:#"\r" withString:#""];

Answer 2

A probable route to a solution is to use Instruments' Leaks template. By setting the inspection range to start just before this code, then looking at which allocations are still alive after this code has returned, you can see what got leaked, and then look at the leaks' pointer histories and the corresponding stack traces to see exactly what happened.