Positive Lookahead Regex

Question

I have the following regex:

^(?=.{8}$).+

The way I understand this is it will accept 8 of any type of character, followed by 1 or more of any character. I feel I am not grasping how a Positive Lookahead works. Because both sections of the Regex are looking for '.' wouldn't any series of characters fit this?

My question is, how does the positive lookahead effect this regex and what is an example of a matching string?

The following did not match when supplied in the following regex tool:

• 123456781
• (12345678)1
• (12345678)
• (abcdefgh)a
• (abcdefgh)
• abc
• 123

EDIT: Removed first two data entries as I clearly wasn't using the regex tool correctly as they now match with exactly 8 characters.

Answer 1

^(?=.{8}$).+

will match the string

aaaaaaaa

Reasoning:

The content inside of the brackets is a lookahead, since it starts with ?=.

The content inside of a lookahead is parsed - it is not interpreted literally.

Thus, the lookahead only allows the regex to match if .{8}$ would match (at the start of the string, in this case). So the string has to be exactly eight characters then it has to end, as evidenced by $.

Then .+ will match those eight characters.

Answer 2

It is trying to match:

^               # start of line, but...
(?=.{8}$)       # only if it precedes exactly 8 characters and the end of line
.+              # this one matches those 8 characters

and from your input, it should also match these (try this engine with match at line breaks checked):

12345678
abcdefgh