Create NSAttributedString out of NSString with Markdown-like syntax for bold text and links.

gistfile1.m

#if TARGET_OS_IPHONE
@import UIKit;
#define JKColor UIColor
#define JKFont UIFont
#else
@import AppKit;
#define JKColor NSColor
#define JKFont NSFont
#endif

@interface NSString (JKAttributer)

/**
 A very minimalist partial implementation of parsing Markdown-style links and strong emphasis texts.
 
 Supports exactly two things: hyperlinks in the inline style [link](http://example.com), and **bold text**. Links are turned into URL-s, and strong font attribute is added to the strong parts.
 
 Supports only the “happy path.” Is not a comprehensive Markdown implementation.
 
 */
- (NSAttributedString *)attributedStringFromLinksAndStrongTextWithBaseFont:(JKFont *)baseFont strongFont:(JKFont *)strongFont urlColor:(JKColor *)urlColor;

@end

@implementation NSString (JKAttributer)

- (NSAttributedString *)attributedStringFromLinksAndStrongTextWithBaseFont:(JKFont *)baseFont strongFont:(JKFont *)strongFont urlColor:(JKColor *)urlColor
{
    __block NSString *s = [self copy];
    
    // Part 1. Process URL-s. The outcome is a plaintext string + URL ranges and their targets
    
    NSError *error = NULL;
    NSRegularExpression *linkRegex = [NSRegularExpression regularExpressionWithPattern:@"\\[([^]]+)\\]\\(([^\\)]+)\\)" options:0 error:&error];
    
    NSMutableArray *linkRanges = [NSMutableArray array];
    NSMutableArray *linkTargets = [NSMutableArray array];
    
    // How much to offset the following match ranges
    __block NSInteger offset = 0;
    
    [linkRegex enumerateMatchesInString:self options:0 range:NSMakeRange(0, self.length) usingBlock:^(NSTextCheckingResult *result, NSMatchingFlags flags, BOOL *stop) {
        
        NSRange range = [result rangeAtIndex:0];
        range.location += offset;
        NSRange urlRangeInTargetString = [result rangeAtIndex:1];
        urlRangeInTargetString.location += offset - 1;
        s = [s stringByReplacingCharactersInRange:range withString:[self substringWithRange:[result rangeAtIndex:1]]];
        [linkRanges addObject:[NSValue valueWithRange:urlRangeInTargetString]];
        [linkTargets addObject:[self substringWithRange:[result rangeAtIndex:2]]];
        
        offset -= [result rangeAtIndex:2].length + 4;
    }];
    
    
    
    // Part 2. Process **strong** parts
    
    NSString *strongInput = [s copy];
    
    __block NSInteger strongOffset = 0;
    NSMutableArray *strongRanges = [NSMutableArray array];
    NSRegularExpression *strongRegex = [NSRegularExpression regularExpressionWithPattern:@"\\*\\*([^*]+)\\*\\*" options:0 error:nil];
    
    [strongRegex enumerateMatchesInString:strongInput options:0 range:NSMakeRange(0, strongInput.length) usingBlock:^(NSTextCheckingResult *result, NSMatchingFlags flags, BOOL *stop) {
        NSRange rangeInString = [result rangeAtIndex:1];
        rangeInString.location += strongOffset - 2;
        [strongRanges addObject:[NSValue valueWithRange:rangeInString]];
        
        NSRange needleRange = [result rangeAtIndex:0];
        needleRange.location += strongOffset;
        NSRange replacementRange = [result rangeAtIndex:1];
        
        s = [s stringByReplacingCharactersInRange:needleRange withString:[strongInput substringWithRange:replacementRange]];
        
        // Shift URL-s back by a step
        for (NSInteger i = 0; i < linkRanges.count; i++) {
            NSValue *value = linkRanges[i];
            NSRange urlRange;
            [value getValue:&urlRange];
            if (urlRange.location > rangeInString.location) {
                urlRange.location -= 4;
                [linkRanges replaceObjectAtIndex:i withObject:[NSValue valueWithRange:urlRange]];
            }
        }
        
        strongOffset -= 4;
    }];
    
    
    
    // Part 3. Compose the output.
    
    NSMutableAttributedString *attributedString = [[NSMutableAttributedString alloc] initWithString:s attributes:
  @{
    NSFontAttributeName: baseFont
    }];
    
    [attributedString beginEditing];
    for (NSValue *strongValue in strongRanges) {
        NSRange strongRange;
        [strongValue getValue:&strongRange];
        [attributedString addAttribute:NSFontAttributeName value:strongFont range:strongRange];
    }
    
    for (NSInteger i = 0; i < linkRanges.count; i++) {
        NSString *urlString = linkTargets[i];
        NSValue *urlRangeValue = linkRanges[i];
        NSRange urlRange;
        [urlRangeValue getValue:&urlRange];
        [attributedString addAttributes:@{
                                          NSLinkAttributeName: [NSURL URLWithString:urlString],
                                          NSForegroundColorAttributeName: urlColor,
                                          NSFontAttributeName: strongFont
                                          } range:urlRange];
    }
    [attributedString endEditing];
    
    return attributedString;
}

@end