Strings & Regular Expressions

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String Basics

const str = "Hello, World!";

str.length;           // 13
str[0];               // "H"
str.at(-1);           // "!" (ES2022)
str.charAt(0);        // "H"
str.charCodeAt(0);    // 72 (Unicode code point)
String.fromCharCode(72); // "H"

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String Methods (Non-Mutating — strings are immutable)

const s = "  Hello World  ";

// Case
s.toLowerCase();          // "  hello world  "
s.toUpperCase();          // "  HELLO WORLD  "

// Trim
s.trim();                 // "Hello World"
s.trimStart();            // "Hello World  "
s.trimEnd();              // "  Hello World"

// Search
s.includes("World");      // true
s.startsWith("Hello");    // false (has leading spaces)
s.trim().startsWith("Hello"); // true
s.endsWith("  ");         // true
s.indexOf("o");           // 5
s.lastIndexOf("o");       // 8
s.search(/world/i);       // 8 (regex, returns index)

// Extract
s.slice(2, 7);            // "Hello"
s.slice(-6, -2);          // "Worl"
s.substring(2, 7);        // "Hello" (no negatives)

// Replace
s.replace("World", "JS");       // "  Hello JS  " (first match)
s.replace(/o/g, "0");           // all matches with regex
s.replaceAll("o", "0");         // ES2021: replace all

// Split
"a,b,c".split(",");       // ["a","b","c"]
"hello".split("");        // ["h","e","l","l","o"]
"a b  c".split(/\s+/);    // ["a","b","c"]

// Pad (ES2017)
"5".padStart(3, "0");     // "005"
"Hi".padEnd(5, ".");      // "Hi..."

// Repeat
"ha".repeat(3);           // "hahaha"

// Match (regex)
"cat bat hat".match(/[a-z]at/g); // ["cat","bat","hat"]

// matchAll (ES2020) — all matches with capture groups
const text = "cat bat hat";
[...text.matchAll(/([a-z])at/g)].map(m => m[1]); // ["c","b","h"]

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Template Literals (Advanced)

// Tagged templates
function sql(strings, ...values) {
  return strings.reduce((acc, str, i) =>
    acc + str + (values[i] !== undefined
      ? `'${String(values[i]).replace(/'/g, "''")}'`
      : ""), "");
}

const name = "O'Brien";
const query = sql`SELECT * FROM users WHERE name = ${name}`;
// SELECT * FROM users WHERE name = 'O''Brien'

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Regular Expressions

// Creating a RegExp
const re1 = /pattern/flags;              // literal (preferred)
const re2 = new RegExp("pattern", "flags"); // dynamic (use for variables)

// Flags
// i — case insensitive
// g — global (find all matches, not just first)
// m — multiline (^ and $ match line boundaries)
// s — dotAll (. matches newlines too)
// u — unicode

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Regex Patterns Reference

Pattern	Meaning
.	Any character except newline
\d	Digit [0-9]
\D	Non-digit
\w	Word char [a-zA-Z0-9_]
\W	Non-word char
\s	Whitespace
\S	Non-whitespace
^	Start of string/line
$	End of string/line
*	0 or more
+	1 or more
?	0 or 1 (optional)
{n}	Exactly n times
{n,m}	Between n and m times
[abc]	Any of a, b, c
[^abc]	None of a, b, c
(abc)	Capture group
(?:abc)	Non-capture group
a|b	a or b
\b	Word boundary

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Common Regex Patterns

const emailRegex = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
const phoneRegex = /^\+?[1-9]\d{9,14}$/;
const dateRegex = /^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$/;

// Named capture groups (ES2018)
const dateMatch = "2024-03-25".match(
  /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/
);
dateMatch.groups.year;   // "2024"
dateMatch.groups.month;  // "03"
dateMatch.groups.day;    // "25"

// Lookahead / Lookbehind
/\d+(?= dollars)/.test("100 dollars"); // positive lookahead: true
/\d+(?! dollars)/.test("100 euros");   // negative lookahead: true
/(?<=\$)\d+/.test("$100");             // positive lookbehind: true

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MCQ — Strings & Regex

Q1: What does "hello".includes("ell") return?

A) 1 (the index) B) true ✅ C) "ell" D) false

Answer: B

Q2: What does "a,b,,c".split(",") return?

A) ["a","b","c"] B) ["a","b","","c"] ✅ C) ["a","b","c",""] D) Error

Answer: B — The empty string between ,, becomes "" in the array.

Q3: What flag makes a regex case-insensitive?

A) g B) m C) i ✅ D) s

Answer: C

Q4: What does "hello".padStart(8, "*") return?

A) "hello***" B) "***hello" ✅ C) "**hello**" D) "hello"

Answer: B — padStart pads to the LEFT until total length is 8.

Q5: Are JavaScript strings mutable?

A) Yes B) No — all string methods return NEW strings ✅

Answer: B

Q6: What does \d+ match in regex?

A) A single digit B) One or more digits ✅ C) Zero or more digits D) A decimal number

Answer: B — \d matches one digit, + means one or more.