regex alphanumeric, but when it gets a numeric, then only numeric

Question

I need to validate an input that must start with an alpha, and then it can be alphanumeric, but once numeric; it must be numeric to the end of the string.

[a-z][a-z,0-9]{1,5}

This does only part of the job. So it validates correctly for

a1
abc12
ab123

but I do not want

a1b2c1

so onces it gets a numeric, the rest must be numeric.

Answer 1

Try this:

^(?=.{2,6}$)([a-z]+[0-9]*)$

First check for 2-6 characters from beginning to end of line. It doesn't even matter what characters they are - you are just checking for length.

Then, 1 or more letters followed by any number of numbers. Since you already checked for 2-6 characters only you don't really care how many letters are followed by how many numbers. At first, I thought it would be much more complicated to list all the possibilities but the positive lookahead does alot of the work

See https://regex101.com/r/HYQIf6/5

Answer 2

This should work for a string of any length:

^[a-z]+([a-z]*|\d*)$

This will return true if the string:

• starts with one or more letters from a to z
• followed by zero or more: letters or numbers until the end

See the matches at Regex101

Edit:

This works as well:

^[a-z]+\d*$

See new regex