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 don't need Regex for this (as for many other common tasks).

public static string AbsolutePathToSubdomain(this Uri uri, string subdomain)
{
    // Pre-process the new subdomain
    if (subdomain == null || subdomain.Equals("www", StringComparison.CurrentCultureIgnoreCase))
        subdomain = string.Empty;

    // Count number of TLDs (assume at least one)
    List<string> parts = uri.Host.Split('.').ToList();
    int tldCount = 1;
    if (parts.Count >= 2 && parts[parts.Count - 2].Length <= 3)
    {
        tldCount++;
    }

    // Drop all subdomains
    if (parts.Count - tldCount > 1)
        parts.RemoveRange(0, parts.Count - tldCount - 1);

    // Add new subdomain, if applicable
    if (subdomain != string.Empty)
        parts.Insert(0, subdomain);

    // Construct the new URI
    UriBuilder builder = new UriBuilder(uri);
    builder.Host = string.Join(".", parts.ToArray());
    builder.Path = "/";
    builder.Query = "";
    builder.Fragment = "";

    return builder.Uri.ToString();
}

Answer 2

^(?:[^.]*\.[^.]*){3}$

the regex above will match the string that has exactly 3 dots --- http://rubular.com/r/Tsaemvz1Yi.

^(?:[^.]*\.[^.]*){4,}$

and this one - for the string that has 4 dots or more --- http://rubular.com/r/IJDeQWVhEB

Answer 3

In Python (excuse me; but regexes are without language frontier)

import re

regx = re.compile('^([^.]*?\.){3}[^.]*?\.')

for ss in ("aaa.bbb.ccc",
           "aaa.bbb.ccc.ffffd",
           'aaa.bbb.ccc.ffffd.eee',
           'a.b.c.d.e.f.g.h.i...'):
  if regx.search(ss):
    print ss + '     has at least 4 dots in it'
  else:
    print ss + '     has a maximum of 3 dots in it'

result

aaa.bbb.ccc     has a maximum of 3 dots in it
aaa.bbb.ccc.ffffd     has a maximum of 3 dots in it
aaa.bbb.ccc.ffffd.eee     has at least 4 dots in it
a.b.c.d.e.f.g.h.i...     has at least 4 dots in it

This regex' pattern doesn't require that the entire string be analysed (no symbol $ in it). It's better on long strings.

Answer 4

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