Generates a regular expression to match against multiple strings.

WordRegexGen.m

#import <Foundation/Foundation.h>

typedef enum {
    CaseInsensitive = 1 << 0,
    DiacraticInsensitive = 1 << 1,
    CanEarlyTerminate = 1 << 2,
} Options;

@interface NSString (Diacritics)

@end

@implementation NSString (Diacritics)

- (NSArray *)diacritics {
    NSString * normalized = [self decomposedStringWithCanonicalMapping];
    if (![self isEqualToString:normalized] && [normalized length] > 0) {
        return @[self, [normalized substringToIndex:1]];
    }
    return @[self];
}

@end

@interface SubstringNode : NSObject

@property (assign) BOOL terminates;
@property (strong) NSMutableDictionary * children;

@end

@implementation SubstringNode

- (id)init {
    self = [super init];
    if (self) {
        _children = [[NSMutableDictionary alloc] init];
    }
    return self;
}

- (id)initWithWords:(NSArray *)words options:(Options)options {
    self = [self init];
    if (self) {
        for (NSString * word in words) {
            [self addWord:word options:options];
        }
        [self optimizeWithOptions:options];
    }
    return self;
}

- (void)addWord:(NSString *)word options:(Options)options {
    if ([word length] == 0) {
        self.terminates = YES;
    }
    else {
        NSRange range = [word rangeOfComposedCharacterSequenceAtIndex:0];
        NSString * character = [word substringWithRange:range];
        character = [NSRegularExpression escapedPatternForString:character];
        if (options & CaseInsensitive)
            character = [character lowercaseString];
        
        NSArray * keys = nil;
        if (options & DiacraticInsensitive) {
            keys = [character diacritics];
        }
        else {
            keys = @[character];
        }
        
        for (NSString * key in keys) {
            SubstringNode * next = [self.children objectForKey:key];
            if (!next) {
                next = [[SubstringNode alloc] init];
                [self.children setObject:next forKey:key];
            }
            [next addWord:[word substringFromIndex:range.length] options:options];
        }
    }
}

- (void)optimizeWithOptions:(Options)options {
    if ([self.children count] > 1) {
        __block BOOL equalChildren = YES;
        SubstringNode * child = [[self.children allValues] lastObject];
        NSString * regex = [child regularExpressionWithOptions:options];
        NSMutableString * newKey = [[NSMutableString alloc] init];
        [self.children enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
            if (![regex isEqualToString:[obj regularExpressionWithOptions:options]] || [obj terminates] != child.terminates) {
                equalChildren = NO;
                *stop = YES;
            }
            [newKey appendString:key];
        }];
        
        if (equalChildren) {
            self.children = [NSMutableDictionary dictionaryWithObject:child forKey:[[NSString alloc] initWithFormat:@"[%@]", newKey]];
        }
    }
    
    [self.children enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
        [obj optimizeWithOptions:options];
    }];
}

- (NSString *)description {
    NSMutableString * output = [NSMutableString string];
    if (self.terminates)
        [output appendString:@"**\n"];
    
    NSRegularExpression * regex = [NSRegularExpression regularExpressionWithPattern:@"^.*$" options:NSRegularExpressionAnchorsMatchLines error:nil];
    [self.children enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop){
        NSString * temp = [obj description];
        [output appendFormat:@"%@:\n", key];
        [regex enumerateMatchesInString:temp options:0 range:NSMakeRange(0, [temp length]) usingBlock:^(NSTextCheckingResult *result, NSMatchingFlags flags, BOOL *stop){
            [output appendFormat:@"|    %@\n", [temp substringWithRange:[result rangeAtIndex:0]]];
        }];
    }];
    
    return output;
}

- (NSString *)regularExpressionWithOptions:(Options)options {
    if (!self.terminates && [self.children count] == 1) {
        return [NSString stringWithFormat:@"%@%@", [[self.children allKeys] lastObject], [[[self.children allValues] lastObject] regularExpressionWithOptions:options]];
    }
    else if (!self.terminates) {
        NSMutableString * regex = [NSMutableString string];
        [self.children enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop){
            if ([regex length] > 0)
                [regex appendString:@"|"];
            [regex appendFormat:@"%@%@", key, [obj regularExpressionWithOptions:options]];
        }];
        return [NSString stringWithFormat:@"(%@)", regex];
    }
    else if ([self.children count] > 0 && !(options & CanEarlyTerminate)) {
        NSMutableString * regex = [NSMutableString string];
        [self.children enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop){
            if ([regex length] > 0)
                [regex appendString:@"|"];
            [regex appendFormat:@"%@%@", key, [obj regularExpressionWithOptions:options]];
        }];
        return [NSString stringWithFormat:@"(%@)?", regex];
    }
    
    return @"";
}

+ (NSString *)regularExpressionWithWords:(NSArray *)words options:(Options)options {
    SubstringNode * root = [[SubstringNode alloc] initWithWords:words options:options];
    return [root regularExpressionWithOptions:options];
}

@end

int main(int argc, char *argv[]) {
    @autoreleasepool {
        NSLog(@"%@", [SubstringNode regularExpressionWithWords:@[@"Hello", @"hi", @"world"] options:CaseInsensitive]);
    }
}