Q: "What are the most common regex character classes?"

"\\d matches a digit (0-9). \\w matches a word character (a-z, A-Z, 0-9, _). \\s matches whitespace (space, tab, newline). \\D, \\W, \\S match the opposite. [abc] matches a, b, or c. [^abc] matches anything except a, b, c."

Question 1

"What is a regex?"

Accepted Answer

"A regular expression (regex) is a sequence of characters that defines a search pattern. It is used to find, validate, or replace text—for example, to match email addresses, numbers, or specific words. Patterns use special characters (e.g. *, +, ?, \d, []) to mean "zero or more", "one or more", "optional", "digit", or "character class". This tool lets you type a regex and test it against your own text."

Question 2

"What regex flags are supported?"

Accepted Answer

"g (global), i (ignore case), m (multiline), u (unicode), s (dotall). Enter them in the Flags field. Duplicates are ignored."

Question 3

"Why do I get 'Invalid flags'?"

Accepted Answer

"Only g, i, m, s, u, y are allowed. If you see duplicate flags (e.g. gg), the tool normalizes them automatically."

Question 4

"How do I replace all matches?"

Accepted Answer

"Enter your pattern and flags, type the replacement text in the Replacement field, then click Replace All. Use $1, $2 for capture groups."

Question 5

"What are the most common regex character classes?"

Accepted Answer

"\d matches a digit (0-9). \w matches a word character (a-z, A-Z, 0-9, _). \s matches whitespace (space, tab, newline). \D, \W, \S match the opposite. [abc] matches a, b, or c. [^abc] matches anything except a, b, c."

Question 6

"What are anchors in regex?"

Accepted Answer

"^ matches the start of a string (or line with the m flag). $ matches the end. \b matches a word boundary (between \w and \W). Example: ^hello$ matches only the string 'hello' and nothing else."

Question 7

"What are quantifiers?"

Accepted Answer

"* matches 0 or more times. + matches 1 or more. ? matches 0 or 1 (makes the preceding element optional). {n} matches exactly n times. {n,m} matches between n and m times. {n,} matches n or more."

Question 8

"What is the difference between greedy and lazy matching?"

Accepted Answer

"Greedy quantifiers (*, +, {n,}) match as much as possible. Lazy quantifiers (*?, +?, {n,}?) match as little as possible. Example: on '', \u003c.*\u003e matches the whole string (greedy), while \u003c.*?\u003e matches '' then '' (lazy)."

Question 9

"What are capture groups and how do I reference them?"

Accepted Answer

"Parentheses create capture groups: (abc). Group 1 is $1 in replacements and \1 in patterns. (?:abc) is a non-capturing group (useful for grouping without creating a reference). Named groups: (?abc) capture into $."

Question 10

"What is a lookahead and lookbehind?"

Accepted Answer

"A lookahead (?=pattern) asserts that what follows matches pattern without consuming it. A negative lookahead (?!pattern) asserts it does not match. Lookbehind (?\u003c=pattern) checks what precedes. Example: \d+(?= dollars) matches numbers followed by ' dollars'."

Question 11

"What is the difference between test() and match() in JavaScript?"

Accepted Answer

"regex.test(str) returns true/false — use it when you only need to know if there is a match. str.match(regex) returns an array of matches (or null). With the g flag, match() returns all matches; without it, it returns the first match with capture groups."

Question 12

"What are common regex patterns for validation?"

Accepted Answer

"Email (basic): ^[\w.-]+@[\w.-]+\.\w{2,}$. URL: ^https?://[\S]+$. Phone (US): ^\+?1?[-.\s]?$?\d{3}$?[-.\s]?\d{3}[-.\s]?\d{4}$. Zip code (US): ^\d{5}(-\d{4})?$. These are simplified — real validation often requires more nuance."

Question 13

"What causes catastrophic backtracking?"

Accepted Answer

"Catastrophic backtracking occurs when a regex engine tries exponentially many combinations on a non-matching string. Example: (a+)+ on 'aaaaab'. Avoid nested quantifiers on overlapping patterns. Use atomic groups or possessive quantifiers where supported."

Question 14

"What does the dotall (s) flag do?"

Accepted Answer

"By default, the dot (.) matches any character except a newline. With the s (dotall) flag, dot also matches newlines. This is useful for matching HTML tags or multi-line strings that span line boundaries."

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