Regular Expression Postive Lookahead substring

Question

I am fairly new to regular expressions and the more and more I use them, the more I like them. I am working on a regular expression that must meet the following conditions:

1. Must start with an Alpha character
2. Out of the next three characters, at least one must be an Alpha character.
3. Anything after the first four characters is an automatic match.

I currently have the following regex: ^[a-zA-Z](?=.*[a-zA-Z]).{1}.*$

The issue I am running into is that my positive lookahead (?=.*[a-zA-Z]).{1} is not constrained to the next three characters following the alpha character.

I feel as if I am missing a concept here. What am I missing from this expression?

Thanks all.

Answer 1

You'll probably have to do a workaround. Something like:

^[a-z](?=([a-z]..|.[a-z].|..[a-z])).{3}.*

• First char [a-z]
• Positive lookahead, either first, or second, or third char is a-z ([a-z]..|.[a-z].|..[a-z])
• Other stuff

Answer 2

The .* in your lookahead is doing that. You should limit the range here like

^[a-zA-Z](?=.{0,2}[a-zA-Z]).{1}.*$

Edit: If you want to make sure, that there are a least 4 characters in the string, you could use another lookahead like this:

 ^[a-zA-Z](?=.{3})(?=.{0,2}[a-zA-Z]).{1}.*$

Answer 3

What do you want lookahead for? Why not just use

^[a-zA-Z](..[a-zA-Z]|.[a-zA-Z].|[a-zA-Z]..)

and be happy?

Answer 4

Change the * in your lookahead to ? to get m/^[a-zA-Z](?=.?[a-zA-Z]).{1}.*$

If I am understanding your criteria, that fixes it because of the change in greediness.

These are correctly matched:

a2a3-match
2aaa-no match
Aaaa-match
a333-no match