([A-zA-Z0-9 _-]) represents what?

Regular Expression, ([a-z] [A-Z] [0-9]) represents three characters, but ([a-zA-Z0-9 _-]) Why is there no length limit? Can abc be used, or can abcdef be used?

Regular Expression, ([a-z] [A-Z] [0-9]) represents three characters, but ([a-zA-Z0-9 _-]) Why is there no length limit? Can abc be used, or can abcdef be used?

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Regular Expression, ([a-z] [A-Z] [0-9]) represents three characters, but ([a-zA-Z0-9 _-]) Why is there no length limit? Can abc be used, or can abcdef be used?

In the original po Image

Used/[a-zA-Z0-9]@gmail\.com/iMatching2abcDEF123@gmail.com

Because the regular expression does not match the beginning or end of a string^And$

So it is equivalent to: only matching in the string3@gmail.comPart

Match the regular expression of the complete email string:

/^[a-z0-9_-]+@[a-z0-9_-]+\.[a-z]+$/i

It's a substring match. If it matches the entire string, it should be written/^[a-zA-Z0-9_-]@gmail\.com$/i.

Square brackets in regular expressions refer to the"One character".

1. [a-z]YesLowercase lettersInOne character
2. [A-Z]YesUppercase lettersInOne character
3. [0-9]YesNumberInOne character
4. [a-zA-Z0-9_-]YesUppercase/lowercase letters, numbers, underscores, and hyphensInOne character
5. [a-z][A-Z][0-9]YesThree charactersThe first is a lowercase letter, the second is a capital letter, and the third is a number.