Confusion on Javascript regex with match function

Question

Why is the third element in the array just '.1' rather than '4.5.1'? I thought that the \d+ would correspond to '3' and the (\.\d)* would capture the remaining decimals and numbers.

var re = /see (chapter \d+(\.\d)*)/i;
var str = 'For more information on regular expressions, see Chapter 3.4.5.1 and CHAPTER 2.3';
    
console.log(str.match(re));

Output:

[ 'see Chapter 3.4.5.1',
  'Chapter 3.4.5.1',
  '.1',
  index: 45,
  input: 'For more information on regular expressions, see Chapter 3.4.5.1 and CHAPTER 2.3' ]

CertainPerformance · Accepted Answer · 2018-09-10 04:01:44Z

2

A repeated capturing group will only capture its last repetition. If you wanted to capture all of the numbers and periods, you should repeat inside the group:

var re = /see (chapter \d+((?:\.\d)*))/i;
var str = 'For more information on regular expressions, see Chapter 3.4.5.1 and CHAPTER 2.3';
    
console.log(str.match(re));

If you plug in your original code into regex101, you'll see a warning describing this:

https://regex101.com/r/uDTcTC/1

A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data

answered Sep 10, 2018 at 4:01

CertainPerformance

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Comments

Leandro Keen Zapa · Accepted Answer · 2018-09-10 04:07:54Z

0

array[0] is a full match
array[1] is a group match caused by a wider parenthesis (chapter \d+(\.\d)*)
array[2] is a group match caused by the narrow parenthesis  (\.\d)*

answered Sep 10, 2018 at 4:07

Leandro Keen Zapa

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