capturing a string before and after some data using regular expressions in ObjectiveC - objective-c

Question

I am relatively new to regex expressions and needed some advice.
The goal is to the get data in the following format into an array:
value=777
value=888
From this data: "value=!##777!##value=##$888*"
Here is my code (Objective C):
NSString *aTestString = #"value=!##777!##value=##$**888***";
NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:#"value=(?=[^\d])(\d)" options:0 error:&anError];
So my questions are:
1) Can the regex engine capture data that is split like that? Retrieving the "value=" removing the garbage data in the middle, and then grouping it with its number "777" etc?
2) If this can be done, then is my regex expression valid? value=(?=[^\d])(\d)

Answer 1

The lookahead (?=) is wrong here, you haven't correctly escaped the \d (it becomes \\d) and last but not least you left out the quantifiers * (0 or more times) and + (1 or more times):
NSString *aTestString = #"value=!##777!##value=##$**888***";
NSRegularExpression *regex = [NSRegularExpression
regularExpressionWithPattern:#"value=[^\\d]*(\\d+)"
options:0
error:NULL
];
[regex
enumerateMatchesInString:aTestString
options:0
range:NSMakeRange(0, [aTestString length])
usingBlock:^(NSTextCheckingResult *result, NSMatchingFlags flags, BOOL *stop) {
NSLog(#"Value: %#", [aTestString substringWithRange:[result rangeAtIndex:1]]);
}
];
Edit: Here's a more refined pattern. It catches a word before =, then discards non-digits and catches digits afterwards.
NSString *aTestString = #"foo=!##777!##bar=##$**888***";
NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:#"(\\w+)=[^\\d]*(\\d+)" options:0 error:NULL];
[regex
enumerateMatchesInString:aTestString
options:0
range:NSMakeRange(0, [aTestString length])
usingBlock:^(NSTextCheckingResult *result, NSMatchingFlags flags, BOOL *stop) {
NSLog(
#"Found: %#=%#",
[aTestString substringWithRange:[result rangeAtIndex:1]],
[aTestString substringWithRange:[result rangeAtIndex:2]]
);
}
];
// Output:
// Found: foo=777
// Found: bar=888

Answer 2

Regular expresssions are expressions that match a given pattern. A regular expression could match, say, a string like "value=!##777" using an expression like "value=[##!%^&][0-9]", which says to match the literal "value=", and then any string made up of the characters #, #, !, %, ^, and &, and finally any string made up of digits. But you can't use a single regular expression by itself to get just the parts of the string that you want, i.e. "value=777".
So, one solution would be to create an expression that recognizes strings such as "value=!##777", and then do some further processing on that string to remove the offending characters.
I think you'll be better off using NSScanner to scan the data and extract the parts you want. For example, you can use -scanString:intoString: to get the "value=" part, followed by -scanCharactersFromSet:intoString: to remove the part you don't want, and then call that method again to get the collection of digits.