RegEx to check for string with given length containing special sequence

Question

I'm looking for a regex that matches strings with a given length (parameterized) that start with "+" or a lowercase letter. It additionally must contain at least one uppercase letter followed by a digit and it must not end with a digit. In between there can be lower and uppercase letters as well as digits [a-zA-Z0-9]. This string may be part of a larger string.

I've got difficulties implementing the length restriction. Tried to solve it with a lookahead but it won't work. Let's say the string's length shall be 10:

(?!.{10,})[a-z\+][a-zA-Z0-9]*([A-Z][0-9])+[a-zA-Z0-9]*[a-zA-Z]

Lengtt of 10:

These example strings should be matched:

c4R9vMh0Lh

+lKj9CnR5x

These example strings should not be matched:

9kR7alcjaa

+5kl9Rk9XZ

aBikJ6clo9

Length of 4:

These example strings should be matched:

aR3v

+K7Z

These example strings should not be matched:

9R3v

+7KZ

aK79

Can you give me some hints?

We're not here to solve homework, but this will help: gskinner.com/RegExr — Camilo Martin
– Camilo Martin, Commented Nov 25, 2010 at 0:02
Can you provide like 5 tests? For some reason I'm too brain dead to generate them, yet I find I'm coherent enough to write a regex (go figure) EDIT Also, please include a couple that shouldn't pass (but may be close) — Brad Christie
– Brad Christie, Commented Nov 25, 2010 at 0:48
Edited my post to add some example strings that should match / should not match. — bin4ry
– bin4ry, Commented Nov 25, 2010 at 9:49

Alan Moore · Accepted Answer · 2010-11-26 00:49:54Z

Kind of a strange requirement, but this seems to do what you want:

/[a-z+]
 (?=([A-Za-z0-9]{8}[A-Za-z]))
 (?=.{0,6}[A-Z][0-9])
 \1
/x

After matching the first character in the normal way, it uses a lookahead to check the length and basic consistency requirements (all letters and digits, doesn't end with a digit). Whatever is matched by the lookahead is captured in group #1.

Then, starting again from the position following the first character, another lookahead checks for the more specific condition: an uppercase letter followed by a digit. If that succeeds, the backreference (\1) goes ahead and consumes the characters that were captured in the first lookahead.

Parameterizing the regex is a simple matter of replacing the numbers inside the braces with numbers or expressions based on the desired length. Here's an example in Java:

import java.util.regex.*;

public class Test
{
  public static void main(String[] args) throws Exception
  {
    String[] inputs = {
      "c4R9vMh0Lh",
      "+lKj9CnR5x",
      "9kR7alcjaa",
      "+5kl9Rk9XZ",
      "aBikJ6clo9",
      "aR3v",
      "+K7Z",
      "9R3v",
      "+7KZ",
      "aK79"
    };

    int len = Integer.parseInt(args[0]);
    String regex = "[a-z+]" +
      "(?=([A-Za-z0-9]{" + (len-2)  + "}[A-Za-z]))" +
      "(?=.{0," + (len-4) + "}[A-Z][0-9])" +
      "\\1";
    Pattern p = Pattern.compile(regex);
    Matcher m = p.matcher("");
    System.out.println("length = " + len);
    System.out.println("regex = " + p.pattern());
    for (String s : inputs)
    {
      System.out.printf("%n%12s : %b%n", s, m.reset(s).find());
    }
  }
}

sample output:

>java Test 4
length = 4
regex = [a-z+](?=([A-Za-z0-9]{2}[A-Za-z]))(?=.{0,0}[A-Z][0-9])\1

  c4R9vMh0Lh : false

  +lKj9CnR5x : true

  9kR7alcjaa : true

  +5kl9Rk9XZ : false

  aBikJ6clo9 : true

        aR3v : true

        +K7Z : true

        9R3v : false

        +7KZ : false

        aK79 : false

This is just marvellous. Thanks so much for your help. Your regex is pretty ingenious. Have a nice first advent!

Paul Creasey · Accepted Answer · 2010-11-25 00:06:05Z

1

You example uses negative look ahead instead of positive, use ^(?=.{10,}) instead. This should work as long as your regex flavour supports look ahead of course.

In my opinion, situations like this are often best with using more than 1 regex, but that is not always an option.

answered Nov 25, 2010 at 0:06

Paul Creasey

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1 Comment

bin4ry Over a year ago

Yea you are right. Additionally i maybe need to find this matching string inside a larger surrounding string. Therefore ^ and $ maybe wrong and i removed it from my example above. Now when i use (?=.{10})[a-z\+][a-zA-Z0-9]*([A-Z][0-9])+[a-zA-Z0-9]*[a-zA-Z] it doesn't match 10 characters long strins but larger ones.

tchrist · Accepted Answer · 2010-11-25 13:59:20Z

This:

#!/usr/bin/perl

$_ = "Hello%20world%20how%20are%20you%20today";
print "<$1>" while m{
    \G ( (?: [^%] | % \p{xdigit}{2} )+ )
    (?:
        (?<= \G .{5} )
       |(?<= \G .{4} )
       |(?<= \G .{3} )
    )
}xg;

Produces this:

<Hello>
<%20wo>
<rld>
<%20ho>
<w%20a>
<re%20>
<you>
<%20to>
<day>

Whereas this:

$_ = <<EOM;
This particularly rapid,
unintelligible patter,
Isn't generally heard,
and if it is it doesn't matter.
EOM

s/(\s)/sprintf("%%%02X", ord $1)/ge;
print "$_\n\n";

produces this:

This%20particularly%20rapid,%20%0Aunintelligible%20patter,%20%0AIsn't%20generally%20heard,%20%0Aand%20if%20it%20is%20it%20doesn't%20matter.%0A

<This>
<%20pa>
<rticu>
<larly>
<%20ra>
<pid,>
<%20>
<%0Aun>
<intel>
<ligib>
<le%20>
<patte>
<r,%20>
<%0AIs>
<n't>
<%20ge>
<neral>
<ly%20>
<heard>
<,%20>
<%0Aan>
<d%20i>
<f%20i>
<t%20i>
<s%20i>
<t%20d>
<oesn'>
<t%20m>
<atter>
<.%0A>

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