Regular expression to validate a string starts with a char and contains allowed chars

Question

I need help with my regular expression written in javascript. I have tried using the regularExpression generator online, and the best i can come up with is the this:

^[a-z.-]{0,50}$

The expression must validate the following

• String first char MUST start with az (no alpha)
• String can contain any char in range az (no alpha), 0-9 and the characters dash "-" and dot "."
• String can be of max length 50 chars

Examples of success strings

• username1
• username.lastname
• username-anotherstring1
• this.is.also.ok

No good strings

• 1badusername
• .verbad
• -bad
• also very bad has spaces

// Thanks

Answer 1

Almost (assuming "no alpha" means no uppercase letters)

https://regex101.com/r/O9hvLP/3

^[a-z]{1}[a-z0-9\.-]{0,49}$

The {1} is optional, I put it there for descriptive reasons

Answer 2

I think this should cover what you want

^[a-z][a-z0-9.-]{0,49}$

That is starts az but then has 0-49 of az , 0-9 or .-

Live example: https://regexr.com/5k8eu

Edit: Not sure if you intended to allow upper and lowercase, but if you did both character classes could add AZ as well!

Answer 3

If the . and - can not be at the end, and there can not be consecutive ones, another option could be:

^[a-z](?=[a-z0-9.-]{0,49}$)[a-z0-9]*(?:[.-][a-z0-9]+)*$

Explanation

• ^ Start of string
• [az] Match a single char az
• (?=[a-z0-9.-]{0,49}$) Assert 0-49 chars to the right to the end of string
• [a-z0-9]* Match optional chars a-z0-9
• (?:[.-][a-z0-9]+)* Optionally match either . or - and 1+ times a char a-z0-9
• $ End of string

Regex demo