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<h3>Introduction</h3><p>Regular expressions (RegExp) are a powerful feature in JavaScript for pattern matching, text validation, searching, and replacement. Once you master basic regex, the next level involves quantifiers, advanced pattern constructs, the RegExp object interface, and the methods you can use. In this guide you’ll explore:</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>Quantifiers and how they control repetition</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>Complex pattern constructs</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>The RegExp object and its properties</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>Regular expression methods to work with strings</li></ol><h2><br></h2><h2>1. Regex Quantifiers</h2><p>Quantifiers specify how many times an element (character, group, class) can repeat in the match. Common quantifiers include:</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x* — zero or more occurrences of x</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x+ — one or more occurrences</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x? — zero or one occurrence</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x{n} — exactly <em>n</em> occurrences</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x{n,} — <em>n</em> or more occurrences</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x{n,m} — at least <em>n</em> and up to <em>m</em> occurrences</li></ol><p>Other variants:</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>Using lazy (non-greedy) matches by appending ? (e.g. x+?, x*?)</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>Using greedy quantifiers by default, which try to match as much as possible</li></ol><p><strong>Examples:</strong></p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>/a{3}/ matches exactly “aaa”</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>/a{2,4}/ can match “aa”, “aaa”, or “aaaa”</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>/a+?/ matches one a even if more are available</li></ol><p>Quantifiers help you craft patterns that match variable-length text precisely.</p><h2><br></h2><h2>2. Advanced Regex Patterns</h2><p>Beyond simple character classes, you can combine constructs to form powerful patterns:</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>Grouping and capturing</strong> using parentheses (...)</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>Non-capturing groups</strong> with (?:...)</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>Character classes & ranges</strong>: [A-Za-z], [0-9], [^xyz]</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>Alternation (OR)</strong> using the pipe |, e.g. cat|dog</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>Nested patterns</strong> combining groups, alternation, quantifiers</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>Escaping special characters</strong> with  when you want literal meaning</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>Backreferences</strong> referencing earlier captured groups by 1, 2, etc.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>Lookahead and lookbehind assertions</strong> (positive and negative)</li></ol><p><strong>Pattern Example:</strong></p><div class="ql-code-block-container" spellcheck="false"><div class="ql-code-block" data-language="plain">let pattern = /^(d{4})-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]d|3[01])$/;</div></div><p>This pattern matches date strings in the format YYYY-MM-DD, where months and days are controlled to valid ranges.</p><h2><br></h2><h2>3. The RegExp Object in JavaScript</h2><p>When you create a regular expression, JavaScript represents it as a RegExp object. It has useful properties and internal behavior:</p><p>Properties of a RegExp object:</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>source — the textual pattern (string)</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>flags — the string of flags used (e.g. "gim")</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>Boolean properties: global, ignoreCase, multiline, unicode, sticky</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>lastIndex — the index at which to start the next match when using the global flag</li></ol><p>You can create a RegExp object in two ways:</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>Literal syntax: /pattern/flags</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>Constructor syntax: new RegExp("pattern", "flags")</li></ol><p>Advantages of the constructor form include dynamic pattern generation (e.g. building the pattern string at runtime).</p><h2><br></h2><h2>4. RegExp Methods</h2><p>JavaScript provides methods both on the RegExp object and on String objects to apply regex patterns.</p><h3>RegExp methods:</h3><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>regex.test(string) — tests whether the pattern matches; returns true or false</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>regex.exec(string) — executes the search and returns an array of match details or null</li></ol><h3>String methods using regex:</h3><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>str.match(regex) — returns an array of matches or null</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>str.replace(regex, replacement) — replaces matched substrings</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>str.search(regex) — returns index of the first match or -1</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>str.split(regex) — splits the string around matches</li></ol><p><strong>Usage Examples:</strong></p><div class="ql-code-block-container" spellcheck="false"><div class="ql-code-block" data-language="plain">let re = /hello/gi;</div><div class="ql-code-block" data-language="plain"><br></div><div class="ql-code-block" data-language="plain">console.log(re.test("Hello world"));  </div><div class="ql-code-block" data-language="plain">// true</div><div class="ql-code-block" data-language="plain"><br></div><div class="ql-code-block" data-language="plain">let matchInfo = re.exec("Hello hello");</div><div class="ql-code-block" data-language="plain">// matchInfo contains details about the match and capture groups</div><div class="ql-code-block" data-language="plain"><br></div><div class="ql-code-block" data-language="plain">console.log("Hello world".match(/o/g));  </div><div class="ql-code-block" data-language="plain">// ["o", "o"]</div><div class="ql-code-block" data-language="plain"><br></div><div class="ql-code-block" data-language="plain">let text = "apple, banana, cherry";</div><div class="ql-code-block" data-language="plain">let parts = text.split(/s*,s*/);</div><div class="ql-code-block" data-language="plain">// ["apple", "banana", "cherry"]</div><div class="ql-code-block" data-language="plain"><br></div><div class="ql-code-block" data-language="plain">let result = "2025-10-12".replace(/-/g, "/");</div><div class="ql-code-block" data-language="plain">// "2025/10/12"</div></div><p>When using global (g) flag, repeated calls to exec() will use lastIndex to find subsequent matches.</p><h2><br></h2><h2>Conclusion</h2><p>With quantifiers, advanced patterns, the RegExp object interface, and methods to integrate with strings, JavaScript’s regular expressions become a flexible and powerful tool. Once you practice, you can validate input, parse formats, replace content, or extract data with confidence.</p>

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