Regex in Objective-C: how to replace matches with a dynamic template?

Question

My input is like "Hi {{username}}", ie. a string with keywords to replace. However, the input is quite small (~ 10 keywords and 1000 characters total), and I have a million possible keywords stored in a hashtable data structure, each associated to its replacement.

Therefore, I do not want to iterate over the keyword list and try to replace each one in the input for obvious performance reason. I prefer to iterate only once over the input characters by looking for the regex pattern "\{\{.+?\}\}".

In Java, I make use of the Matcher.appendReplacement and Matcher.appendTail methods to do that. But I cannot find a similar API with NSRegularExpression.

private String replaceKeywords(String input)
{        
    Matcher m = Pattern.compile("\\{\\{(.+?)\\}\\}").matcher(input);
    StringBuffer sb = new StringBuffer();

    while (m.find())
    {
        String replacement = getReplacement(m.group(1));
        m.appendReplacement(sb, replacement);
    }

    m.appendTail(sb);
    return sb.toString();
}

Am I forced to implement such API myself, or did I miss something?

Answer 1

You can achieve this with NSRegularExpression:

NSString *original = @"Hi {{username}} ... {{foo}}";
NSDictionary *replacementDict = @{@"username": @"Peter", @"foo": @"bar"};

NSString *pattern = @"\\{\\{(.+?)\\}\\}";
NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:pattern
                                       options:0
                                     error:NULL];

NSMutableString *replaced = [original mutableCopy];
__block NSInteger offset = 0;
[regex enumerateMatchesInString:original
            options:0
              range:NSMakeRange(0, original.length)
             usingBlock:^(NSTextCheckingResult *result, NSMatchingFlags flags, BOOL *stop) {
    NSRange range1 = [result rangeAtIndex:1]; // range of the matched subgroup
    NSString *key = [original substringWithRange:range1];
    NSString *value = replacementDict[key];
    if (value != nil) {
        NSRange range = [result range]; // range of the matched pattern
        // Update location according to previous modifications:
        range.location += offset;
        [replaced replaceCharactersInRange:range withString:value];
        offset += value.length - range.length; // Update offset
    }
}];
NSLog(@"%@", replaced);
// Output: Hi Peter ... bar

---

A Swift version using the Regex type that was introduced with Swift 5.7, requires macOS 13+ or iOS 16+):

import Foundation

let original = "Hi {{username}} ... {{foo}}"
let replacementDict = ["username" : "Peter", "foo" : "bar" ]

var replaced = ""
let regex = /\{\{(.+?)\}\}/
var pos = original.startIndex

// Look for next occurrence of {{key}}.
while let match = try? regex.firstMatch(in: original[pos...]) {
    // Append original text preceding the match.
    replaced += original[pos..<match.range.lowerBound]
    
    // Replace {{key}} by corresponding value and append to the output.
    let key = String(match.1)
    if let value = replacementDict[key] {
        replaced += value
    } else {
        replaced += match.0
    }

    // Continue search after the current match.
    pos = match.range.upperBound
}
// Append original text after the final match.
replaced += original[pos...]

print(replaced)
// Hi Peter ... bar