Geek Answers Handbook

Positive Lookahead Expression

I have the following regex:

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

The way I understand it, will take 8 characters of any type, followed by 1 or more characters. I feel like I don’t understand how Positive Lookahead works. Because both sections of Regex are looking. Does any character series match?

My question is, how does a positive lookahead affect this regular expression and what is an example of a matching string?

The following did not match if presented in the following regular expression tool :

  • 123456781
  • (12345678) 1
  • (12345678)
  • (ABCDEFGH) a
  • (ABCDEFGH)
  • Abc
  • 123

EDIT: The first two data records were deleted, as I obviously did not use the regular expression tool correctly, since they now only correspond to 8 characters.

+4
source share
3 answers

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

will match the string

aaaaaaaa

Reasoning:

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

The content within the analyzed analysis is analyzed - it is not interpreted literally.

Thus, lookahead only allows the regular expression to match if .{8}$ matches (in this case, at the beginning of the line). Thus, the string should be exactly eight characters long, then it should end, as evidenced by $ .

Then. + will match these eight characters.

+7
source

He 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 caharacters

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

 12345678 abcdefgh
+5
source

Conformity 12345678 works in ruby:

 '12345678' =~ /^(?=.{8}$).+/ => 0

Perhaps your test site does not support viewing regular expressions?

0
source

More articles:


All Articles