.Net Core Data Annotation clarification - asp.net-core

Question

Can someone please guide which one of the below is the correct Data Annotation, if I want to allow just alphabets:
[RegularExpression(#"[a-zA-Z]*", ErrorMessage = "Invalid {0}")]
OR
[RegularExpression(#"^[a-zA-Z]*", ErrorMessage = "Invalid {0}")]
Both seems to be working. The difference is ^ symbol.

Answer 1

^ Caret is a Position Anchor.
Position Anchors does not match character, but position such as start-of-line, end-of-line, start-of-word and end-of-word.
In this case you need both ^ and $: start-of-line and end-of-line respectively. E.g., ^[0-9]$ matches a numeric string.
So you should go with,
[RegularExpression(#"^[a-zA-Z]*$", ErrorMessage = "Invalid {0}")]
Becase you need strings starts and ends with alphabetical characters only, not having any other characters such as symbols or numerals. Here are some examples that you can play with.
let str1 = 'abcDef';
let str2 = '123abcDef';
let str3 = 'abcDef123';
let str4 = 'abc123Def';
let my_regex = /^[a-zA-Z]*$/;
let your_regex = /[a-zA-Z]*/;
alert(str1 + " : " + my_regex.test(str1) + " with my regex");
alert(str2 + " : " + my_regex.test(str2) + " with my regex");
alert(str3 + " : " + my_regex.test(str3) + " with my regex");
alert(str4 + " : " + my_regex.test(str4) + " with my regex");
alert(str1 + " : " + your_regex.test(str1) + " with your regex");
alert(str2 + " : " + your_regex.test(str2) + " with your regex");
alert(str3 + " : " + your_regex.test(str3) + " with your regex");
alert(str4 + " : " + your_regex.test(str4) + " with your regex");

Answer 2

The Caret (^) character is also referred to by the following terms:
Terminology
hat, control, uparrow, chevron, circumflex accent
Usage
It has two uses in regular expressions:
To denote the start of the line
If used immediately after a square bracket ([^) it acts to negate the set of allowed characters (i.e. [123] means the character 1, 2, or 3 is allowed, whilst the statement [^123] means any character other than 1, 2, or 3 is allowed.
Character Escaping
To express a caret without special meaning, it should be escaped by preceding it with a backslash; i.e. ^.
You can find it here...

Answer 3

I think you want something more like this:
[RegularExpression(#"^[a-zA-Z]+$", ErrorMessage = "Invalid {0}")]
which says to match
the beginning of the string ^
followed by 1 or more alphabetic characters [a-zA-Z]+ (you do need more than zero right)?
followed by the end of the string $
This doesn't allow other strings to match such as 123abc or abc123 because the anchors of ^ and $ prevent that.
In your first example, the match would allow an empty string, and would allow for the cases I mentioned in the paragraph above. Your second example would allow empty string, but would at least filter out 123abc but would still allow abc123 because you don't have the $ marker.
If you want to take my solution and extend it beyond ASCII alphabetic characters, you can change [a-ZA-Z]+ to \p{L}+, which should work universally in Unicode (but that seems like it might be more than you're looking for; just including for completeness).
Finally, [RegularExpression] uses the standard regex capability that has been part of .NET for quite some time, expressed in the Regular Expression Language - Quick Reference.