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<h2 id="groupscapturing">Groups &amp; Capturing</h2>
<p>Grouping is the most useful feature of regex. Grouping can be done by placing regular expression inside round brackets. </p>
<p>It unifies the regular expressions inside it as a single unit. Let's look at its usages one by one:</p>
<ol>
<li><p>It makes the regular expression more readable and sometimes it is an inevitable thing.</p>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuaa" class="embed"></iframe>
</div>
<p>Suppose, we want to match both the sentences in the above text, then grouping is the inevitable thing.</p>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuad" class="embed"></iframe>
</div></li>
<li><p>To apply quantifiers to one or more expressions.</p>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuag" class="embed"></iframe>
</div></li>
</ol>
<p>Similarly, you can use other quantifiers.</p>
<ol>
<li><p>To extract and replace substrings using groups. So, we call groups <strong>Capturing groups</strong>, becuase we are capturing data(substrings) using groups.</p>
<p>In this part, we will see how to extract and replace data using groups in Javascript.</p>
<p><strong>Data Extraction</strong></p>
<p>Observe the code below.</p>
<pre><code class="js language-js">var str = "2020-01-20";
// Pattern string
var pattern = /(\d{4})-(\d{2})-(\d{2})/g;
// ^ ^ ^
//group-no: 1 2 3
var result = pattern.exec(str);
// printing
console.log(result);
/* Output will be:
[
'2020-01-20', //-------pattern
'2020', //-----First group
'01', //-------Second group
'20', //-------Third group
index: 0,
input: '2020-01-20',
groups: undefined
]
*/
// Data extraction
console.log(result[1]); // First group
console.log(result[2]); // Second group
console.log(result[3]); // Third group
</code></pre>
<p>In the output array, the first data is a match string followed by the matched groups in the order.</p>
<p><strong>Data Replacement:</strong></p>
<p><code>Replace</code> is another function, which can be used to replace and rearrange the data using regex. Observe the code below.</p>
<pre><code class="js language-js">var str = "2020-01-20";
// Pattern string
var pattern = /(\d{4})-(\d{2})-(\d{2})/g;
// ^ ^ ^
//group-no: 1 2 3
// Data replacement using $group_no
var ans=str.replace(pattern, '$3-$2-$1');
console.log(ans);
// Output will be: 20-01-2020
</code></pre>
<p>As you can see, we have used <code>$group_no</code> to indicate the capturing group.</p></li>
</ol>
<h2 id="problems">Problems</h2>
<p>Predict the output of the following regex:</p>
<ol>
<li><p><strong>RegEx</strong>: <code>([abc]){2,}(one|two)</code> <br>
<strong>Text</strong>:
<code>aone
cqtwo
abone
actwo
abcbtwoone
abbcccone
</code></p>
<p>Answer: </p>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuaj" class="embed"></iframe>
</div></li>
<li><p><strong>RegEx</strong>: <code>([\dab]+(r|c)){2}</code> <br>
<strong>Text</strong>:
<code>1r2c
ar4ccc
12abr12abc
acac, accaca, acaaca
aaar1234234c, aaa1234234c
194brar, 134bcbb-c </code></p>
<p>Answer: </p>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuam" class="embed"></iframe>
</div></li>
</ol>
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