Matching a regular expression with a string (file name) - objective-c

Question

I'm trying to differentiate between 2 files (in NSString format). As far as I know, this can be done by comparing and matching a regular expression. The format of the 2 jpg files which I have are:
butter.jpg
butter-1.jpg
My question is what regular expression can I write to match the 2 strings above? I've search and found an example expression, but I'm not sure how is it read and think it's wrong.
Here is my code:
NSString *exampleFileName = [NSString stringWithFormat:#"butter-1.jpg"];
NSString *regEx = #".*l{2,}.*";
NSPredicate *regExTest = [NSPredicate predicateWithFormat:#"SELF MATCHES %#", regEx];
if ([regExTest evaluateWithObject:exampleFileName] == YES) {
NSLog(#"Match!");
} else {
NSLog(#"No match!");
}
EDIT:
I tried using the following:
NSString *regEx = #"[a-z]+-[0-9]+.+jpg";
to try to match:
NSString *exampleFileName = [NSString stringWithFormat:#"abcdefg-112323.jpg"];
Tested with:
abc-11.jpg (Match)
abcsdas-.jpg (No Match)
abcdefg11. (No Match)
abcdefg-3123.jpg (Match)
As of now it works, but I want to eliminate any chances that it might not, any inputs?

Answer 1

NSString *regEx = #"[a-z]+-[0-9]+.+jpg";
will fail for butter.jpg, as it needs to have one - and at least on number.
NSString *regEx = #"[a-z]+(-[0-9]+){0,1}.jpg";
and if you do
NSString *regEx = #"([a-z])+(?:-([0-9])+){0,1}.jpg";
You can access the informations you probably would like to have later as capture groups.
(...) |Capturing parentheses. Range of input that matched the parenthesized subexpression is available after the match.
and if you dont need capture groups
NSString *regEx = #"(?:[a-z])+(?:-[0-9]+){0,1}.jpg";
(?:...)| Non-capturing parentheses. Groups the included pattern, but does not provide capturing of matching text. Somewhat more efficient than capturing parentheses.

Answer 2

You can match an alphabetic character (in any language) using \p{L}. You can match a digit using \d. You need to escape the . because in a regular expression, . means “any character”.
Parsing a regular expression is expensive, so you should only do it once.
BOOL stringMatchesMyPattern(NSString *string) {
static dispatch_once_t once;
static NSRegularExpression *re;
dispatch_once(&once, ^{
re = [NSRegularExpression regularExpressionWithPattern:
#"^\\p{L}+-\\d+\\.jpg$" options:0 error:NULL];
}
return nil != [re firstMatchInString:string options:0
range:NSMakeRange(0, string.length)];
}