How to clean the details with regex?

What regex expression can I use to ensure this pattern match (see details of the question)?

• Conditions: 1. First character is between a-z (lowercase) 2. Last character must be either a-z (lowercase) or 0-9 3. Can have underscores anywhere in between 4, Can have nothing outside of a-z (lowercase), 0-9 or underscores in the whole string

• Answer:

  The naive expression ^[a-z][a-z_0-9]*[a-z0-9]$ fails to match for the empty string, or single-character lowercase alphabetical strings. A correct answer is: ^[a-z]?((?<=.)[a-z_0-9])*((?<=.)[a-z0-9])$ where (?<=x) is a positive lookbehind on x. This regular expression is constructed as follows: ^ — match the start of the string; unnecessary in some languages and contexts [a-z]? — match zero or one alphabetical character; this is the first character, if any ((?<=.)[a-z_0-9])* — match zero or more alphabetical, numeric, or underscore characters, as long as they are not the first character in the string ((?<=.)[a-z0-9] — matches the last character, as long as it is not the first character in the string $ — match the end of the string; unnecessary in some contexts If you want to exclude the empty string as a match, you can use this expression: ^[a-z]([a-z_0-9](?=.))*[a-z0-9]?$