Positive lookahead?

[santi77]

I'm trying to get a random password with some restrictions, this is my regex

^(?=.\d)(?=.[a-z])(?=.[A-Z])(?=.[@$._*#&%]).{8,16}$

This regex says:

• have one or more digits
• have lowercase letters
• have uppercase letters
• have one or more that symbols
• lenght between 8 and 16 chars

When I try to run this on Xeger the code hangs, It is possible to do this?

[StefH]

@santi77 - I think your pattern should be like:

string pattern = "^(?.\\d)(?.[a-z])(?.[A-Z])(?.[@$\\._\\*#&%]).{8,16}$";
string value = new Xeger(pattern).Generate();

[santi77]

That works, thank you!!!

[StefH]

I've my doubts if this actually works. lookahead is not implemented

Also when using Rex (https://rise4fun.com/rex), I get this error: The following constructs are currently not supported: anchors \G, \b, \B, named groups, lookahead, lookbehind, as-few-times-as-possible quantifiers, backreferences, conditional alternation, substitution

So in your case, to generate a valid pattern, you can use like this:

"\\d{2,4}[a-z]{2,4}[A-Z]{2,4}[0-9a-zA-Z]{2,4}"

However this is not really random...

[gregsdennis]

This worked for me as well, but I don't understand why. The construct (?=.<something>) works for online regex validators, but (?.<something>) is reported as invalid yet it works in .Net. And it doesn't work for this online .net validator: http://regexstorm.net/tester

Looks like it's an issue with this lib. The first syntax works for validation (Regex) but not for generation (Xeger).

Note also that (?.(dog)) only ever creates string that start with a random char, then "dog", then more randomness. I'll never get "dog" in another position within the generated string, which is what (?=.*(dog)) allows.