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133 lines
5.2 KiB
HTML
133 lines
5.2 KiB
HTML
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<html>
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<head>
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<style type="text/css">
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.container {
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position: static;
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width: 800px;
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height: 350px;
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overflow: hidden;
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}
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.embed {
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height: 100%;
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width: 100%;
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min-width: 1000px;
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margin-left: -360px;
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margin-top: -57px;
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overflow: hidden;
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}
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body {
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width: 800px;
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margin: auto;
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padding: 1em;
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font-family: "Open Sans", sans-serif;
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line-height: 150%;
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letter-spacing: 0.1pt;
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}
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img {
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width: 90%;
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text-align: center;
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margin: auto;
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box-shadow: 0 4px 8px 0 rgba(0, 0, 0, 0.2), 0 6px 20px 0 rgba(0, 0, 0, 0.19);
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}
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pre, code {
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padding: 1em;
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}
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</style>
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<script>
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document.addEventListener('readystatechange', event => {
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if (event.target.readyState === "complete")
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document.activeElement.blur();
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});
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</script>
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<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.18.1/styles/default.min.css">
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<script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.18.1/highlight.min.js"></script>
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</head>
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<body>
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<h2 id="backreferencing">Backreferencing</h2>
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<p>Backreferencing is used to match same text again. Backreferences match the same text as previously matched by a capturing group. Let's look at an example:</p>
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<div class="container">
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<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuap" class="embed"></iframe>
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</div>
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<p><strong>Note:</strong> <code>\/</code> is escaped <code>/</code>character, check it out in the appendix.</p>
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<p>The first captured group is (<code>\w+</code>), now we can use this group again by using a backreference (<code>\1</code>) at the closing tag, which matches the same text as in captured group <code>\w+</code>.</p>
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<p>You can backreference any captured group by using <code>\group_no</code>.</p>
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<p>Let's have two more examples:</p>
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<div class="container">
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<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuas" class="embed"></iframe>
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</div>
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<div class="container">
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<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vube" class="embed"></iframe>
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</div>
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<h3 id="backreferencingandcharacterclass">Backreferencing and character class</h3>
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<p>Backreferencing can not be used in character class. Let's see an example:</p>
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<div class="container">
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<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vu7p" class="embed"></iframe>
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</div>
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<h3 id="backreferencingandquantifiers">Backreferencing and Quantifiers</h3>
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<p>When we are using a backreference for an expression with quantifiers, then we have to be careful. Let's observe it:</p>
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<div class="container">
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<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuet" class="embed"></iframe>
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</div>
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<p>Note that <code>(\d)+</code> and <code>(\d+)</code> both are different. So, what will happen for <code>(\d)+ -- \1</code> expression and same text above?</p>
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<div class="container">
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<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vu85" class="embed"></iframe>
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</div>
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<p>Can you observe something?</p>
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<p>For <code>(\d)+ -- \1</code> expression and <code>123 -- 3</code> string, first time 1 was stored in \1, then 2 was stored in \1 and at last 3 was stored. So, it will show a match if and only if the last character before <code>--</code> is exactly same as the character after <code>--</code>.</p>
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<p><strong>Problems:</strong></p>
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<ol>
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<li><p>Match any palindrome string of length 6, having only lowercase letters.
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Answer: <code>([a-z])([a-z])([a-z])\3\2\1</code></p>
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<div class="container">
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<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuoc" class="embed"></iframe>
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</div></li>
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<li><p><strong>RegEx</strong>: <code>(\w+)oo\1le</code> <br>
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<strong>Text:</strong> <code>google, doodle jump, ggooggle, ssoosle</code></p>
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<p>Answer: </p>
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<div class="container">
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<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vubk" class="embed"></iframe>
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</div></li>
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</ol>
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<p><strong>Note:</strong> For group numbers more than 9, there is a syntax difference.</p>
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<script type="text/javascript">
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document.addEventListener('DOMContentLoaded', (event) => {
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document.querySelectorAll('pre code').forEach((block) => {
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hljs.highlightBlock(block);
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});
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});
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</script>
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</body>
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</html>
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