regex to start and end with different conditions

Question

The following rules apply to the string I need to test:

• must start with only letters(1 or more)
• can contain letters, numbers and/or underscores ( _ ) (0 or more)
• must end with letters or numbers (1 or more)

^[a-z]+[\w]*[a-z0-9]$/gi is the closest I got but it doesn't match a string that contains a single letter.

Examples that should match:

• test_regex_9
• e
• test9

Examples that should not match:

• test_
• _test
• 9test

^[a-z]+[\w]*[a-z0-9]*$/gi also allows the string to end with _

Answer 1

Another way with regex alternation group:

^([a-z]+[\w]*[a-z0-9]|[a-z]+)$

Answer 2

You may use an optional group:

/^[a-z]+(?:\w*[a-z0-9])?$/i

Details

• ^ - start of string
• [a-z]+ - one or more letters
• (?:\w*[a-z0-9])? - one or zero occurrences of
  • \w* - 0 or more word chars
  • [a-z0-9] - one alphanumeric char
• $ - end of string.