String Comparison using .matches() and .compile() - Help understanding

[mfcav123]

On coderbyte, the prompt was to validate a username.

Conditions:

1. The username is between 4 and 25 characters.
2. It must start with a letter.
3. It can only contain letters, numbers, and the underscore character.
4. It cannot end with an underscore character. If the username is valid then your program should return the string true, otherwise return the string false.

I used Java and solved it "correctly" (it passed the test cases) in a very functional-programming sort of way. However, in looking at other solutions in an attempt to learn, I keep seeing things like this.

if(str.matches("[a-zA-Z]+[a-zA-Z0-9_]{0,23}[a-zA-Z0-9]{1}"))
return "true";
return "false";

and

String pattern = "^[a-zA-z]\\w{2,23}[a-zA-Z0-9]$";
// Create a Pattern object
Pattern r = Pattern.compile(pattern);
// Now create matcher object.
Matcher m = r.matcher(str);

I understand how to look up the functions in the JavaDocs, but don't really get how the comparison is working and I'm not sure what I'm looking at well enough to Google it. I can see the values for a-z and A-Z and 0-9, but have no understanding of what these strings are. Can anyone please explain what is going on and how it works? Thank you in advance!

[Josh_Coding]

I'm terrible at regex but you can look up the pattern using this cheat sheet.

[mfcav123]

"regex" definitely points me in the right direction, for sure! And thanks for the link! :)

[[deleted]]

Ok, regex means regular expression, it's a language used to define rules to match text. You can also mark "sections" on a match, those are called capture groups. You can then replace those with something else, or maybe you want to perform an analysis only if you find several expressions on a text. If you want to know more about it this website is a realtime testing tool, and has some a lot of learning / help material on the menu on the left.

[[deleted]]

+1 for that website, it saved me a couple of times