How to conditional regex

Question

I want a regex that does one thing if it has 3 instances of .in the string, and something else if it has more than 3 instances.

for example

Answer 1

You can do this using the (?(?=condition)then|else) construct. However, this is not available in JavaScript (but it is available in .NET, Perl and PCRE):

^(?(?=(?:[^.]*\.){3}[^.]*$)aaa|eee)

for example, will check if a string contains exactly three dots, and if it does, it tries to match aaa at the start of the string; otherwise it tries to match eee. So it will match the first three letters of

aaa.bbb.ccc.ffffd
eee.ffffd.ccc.bbb.aaa
eee

but fail on

aaa.bbb.ccc
eee.ffffd.ccc.bbb
aaa.bbb.ccc.ffffd.eee

Explanation:

^            # Start of string
(?           # Conditional: If the following lookahead succeeds:
 (?=         #   Positive lookahead - can we match...
  (?:        #     the following group, consisting of
   [^.]*\.   #     0+ non-dots and 1 dot
  ){3}       #     3 times
  [^.]*      #     followed only by non-dots...
  $          #     until end-of-string?
 )           #   End of lookahead
 aaa         # Then try to match aaa
|            # else...
 eee         # try to match eee
)            # End of conditional