Javascript - Using Concatenated String in Regex

Question

I'm trying to find if a given string of digits contains a sequence of three identical digits.

using a for loop, each digit in the string gets its own representation of a three digit sequence which is then checked against the string using Regex:

var str = "6854777322"

for(var i=0; i<str.length; i++)
{
   seqToCompare = str[i] + str[i] + str[i];
   var re = new RegExp(seqToCompare, "g");
   if(str.match(re).length == 1)
   {
      match = str[i];
   }
}
console.log(match)

The result should be seven (if I put 777 in seqToCompare, it would work), but it looks like the concatenation causes it to fail. Console shows "cannot read property length for null".

You can test it here - https://jsfiddle.net/kwnL7vLs/

I tried .toString, setting seqToCompare in Regex format and even parsing it as int (out of desperation for not knowing what to do anymore...)

If the match fails, the result is null and on the first iteration, the match does fail. — VLAZ
– VLAZ, Commented Nov 26, 2016 at 17:14
Thanks! I was sure it returned 0 rather than null because a comparison took place. — Nivi M
– Nivi M, Commented Nov 26, 2016 at 17:26

anubhava · Accepted Answer · 2016-11-26 17:14:08Z

3

Rather than looping over each character, you can use a simple regex to get a digit that is repeated 3 times:

/(\d)(?=\1{2})/

(\d) - Here we match a digit and group it in captured group #1
(?=\1{2}) is lookahead that asserts same captured group #1 is repeated twice ahead of current position

RegEx Demo

answered Nov 26, 2016 at 17:14

anubhava

790k67 gold badges603 silver badges671 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Eric Zhang · Accepted Answer · 2016-11-26 17:17:13Z

0

anubhava's answer is the way to go, as it's more efficient and simpler. However, if you're wondering why your code specifically is giving an error, it's because you try to find the length property of the return value of str.match(), even when no match is found.

Try this instead:

var str = "6854777322"

for(var i=0; i<str.length; i++)
{
   seqToCompare = str[i] + str[i] + str[i];
   var re = new RegExp(seqToCompare, "g");
   if(str.match(re))
   {
      match = str[i];
   }
}
console.log(match)

answered Nov 26, 2016 at 17:17

Eric Zhang

6017 silver badges14 bronze badges

Collectives™ on Stack Overflow

Javascript - Using Concatenated String in Regex

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related