Regex Tester with Capture Groups — Debug Patterns in Real-Time

Free online regex tester with capture groups, named groups, flags (g,i,m,s,u,y), substitution, and explanation. Web Worker powered for large inputs. Client-side.

Why Test Regular Expressions?

Regular expressions (regex) are powerful but notoriously hard to write and debug. A single misplaced character changes the entire match behavior. An interactive regex tester lets you see matches in real-time, inspect capture groups, test edge cases, and understand exactly what your pattern matches — before deploying to production.

DevStackIO's Regex Tester provides real-time matching with full capture group visualization, support for all major flags (g, i, m, s, u, y), substitution testing, and regex explanation. All client-side, runs in a Web Worker for large inputs.

Regex Fundamentals

Anatomy of a Pattern

/pattern/flags
 ^      ^    ^
 |      |    └─ Flags: g, i, m, s, u, y
 |      └─ Delimiters (JS: /.../, others: implicit)
 └─ The pattern itself

Common Metacharacters

Char	Meaning	Example	Matches
`.`	Any char (except newline)	`c.t`	cat, cot, cxt
`\d`	Digit `[0-9]`	`\d{3}-\d{4}`	555-1234
`\D`	Non-digit	`\D+`	"abc"
`\w`	Word char `[a-zA-Z0-9_]`	`\w+`	"hello123"
`\W`	Non-word char	`\W+`	"!@#"
`\s`	Whitespace	`\s+`	" \t\n"
`\S`	Non-whitespace	`\S+`	"hello"
`^`	Start of string/line	`^Hello`	"Hello world"
`$`	End of string/line	`world$`	"Hello world"
`\b`	Word boundary	`\bcat\b`	"cat" not "catapult"
`\B`	Non-word boundary	`\Bcat\B`	"scatter"

Quantifiers

Quantifier	Meaning	Example
`*`	0 or more	`ab*c` → ac, abc, abbc, abbbc...
`+`	1 or more	`ab+c` → abc, abbc, abbbc...
`?`	0 or 1	`ab?c` → ac, abc
`{n}`	Exactly n	`a{3}` → aaa
`{n,}`	n or more	`a{2,}` → aa, aaa, aaaa...
`{n,m}`	n to m	`a{2,4}` → aa, aaa, aaaa

Greedy vs Lazy:

Greedy (default): .* matches as much as possible
Lazy: .*? matches as little as possible

// HTML tag example
const html = "<div>Hello</div><span>World</span>";

/<div>.*<\/div>/.exec(html);   // Greedy: "<div>Hello</div><span>World</span>"
/<div>.*?<\/div>/.exec(html);  // Lazy:   "<div>Hello</div>"

Character Classes

Pattern	Description
`[abc]`	a, b, or c
`[a-z]`	a through z
`[^abc]`	NOT a, b, or c
`[a-zA-Z0-9]`	Alphanumeric
`[[:alpha:]]`	POSIX alpha (locale-aware)

Capture Groups

Numbered Groups `()`

const regex = /(\d{4})-(\d{2})-(\d{2})/;
const match = regex.exec("2024-01-15");
/*
match[0] = "2024-01-15"  (full match)
match[1] = "2024"        (year)
match[2] = "01"          (month)
match[3] = "15"          (day)
*/

Named Groups `(?<name>)` (ES2018+)

const regex = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/;
const match = regex.exec("2024-01-15");
/*
match.groups = { year: "2024", month: "01", day: "15" }
match[0] = "2024-01-15"
*/

Non-Capturing Groups `(?:)`

// Group for alternation without capturing
/(?:cat|dog|bird)s/  // Matches "cats", "dogs", "birds"
// No extra capture group created

Backreferences `\1`, `\2`, `\k<name>`

// Match repeated words
/\b(\w+)\s+\1\b/.test("hello hello");  // true
/\b(\w+)\s+\1\b/.test("hello world");  // false

// Named backreference
/(?<word>\w+)\s+\k<word>/.test("test test");  // true

Flags (Modifiers)

Flag	Name	Effect
`g`	Global	Find all matches, not just first
`i`	Case-insensitive	`/cat/i` matches "Cat", "CAT", "cat"
`m`	Multiline	`^` and `$` match line boundaries
`s`	DotAll (ES2018)	`.` matches newlines too
`u`	Unicode (ES2015)	Proper Unicode handling, enables `\p{...}`
`y`	Sticky (ES2015)	Match only at `lastIndex` position

// Unicode property escapes (requires 'u' flag)
/\p{Script=Greek}+/u.test("αβγ");     // true
/\p{Emoji}+/u.test("😀🎉");          // true
/\p{General_Category=Letter}/u.test("你好"); // true

How to Test Regex Online (Step by Step)

Open the tester — DevStackIO Regex Tester
Enter pattern — Type regex (with or without /delimiters/)
Set flags — Toggle g, i, m, s, u, y as needed
Enter test string — Paste text, type, or load file
Real-time results — Matches highlight instantly in test string
Inspect groups — Panel shows full match + all capture groups per match
Test substitution — Enter replacement string (supports $1, $&, $<name>)
Explain mode — Get plain-English breakdown of each pattern part
Copy/export — Pattern, matches, or substitution result

Common Patterns Library

Validation

# Email (simplified, RFC 5322 is complex)
^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)+$

# URL
^https?:\/\/(?:www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b(?:[-a-zA-Z0-9()@:%_\+.~#?&\/=]*)$

# IPv4
^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$

# UUID v4
^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$

# Credit card (Luhn check needs code)
^(?:4[0-9]{12}(?:[0-9]{3})?|5[1-5][0-9]{14}|3[47][0-9]{13}|3(?:0[0-5]|[68][0-9])[0-9]{11}|6(?:011|5[0-9]{2})[0-9]{12})$

# Strong password (8+ chars, upper, lower, digit, special)
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[!@#$%^&*]).{8,}$

Extraction

# URLs in text
https?:\/\/(?:www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b(?:[-a-zA-Z0-9()@:%_\+.~#?&\/=]*)

# Email addresses
[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)+

# IP addresses (v4)
\b(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b

# Hashtags
#\w+

# Mentions
@\w+

# Hex colors
#(?:[0-9a-fA-F]{3}){1,2}\b

# Dates (YYYY-MM-DD)
\d{4}-\d{2}-\d{2}

# Version numbers
v?\d+\.\d+\.\d+(?:-[a-zA-Z0-9.]+)?

Transformation

# CamelCase to snake_case
([a-z])([A-Z]) → $1_$2 (then lowercase)

# Snake_case to CamelCase
_([a-z]) → \U$1 (then capitalize first)

# Remove HTML tags
<[^>]*> → (empty)

# Normalize whitespace
\s+ → (space)

# Add quotes to CSV fields
^|,(?=(?:[^"]*"[^"]*")*[^"]*$) → ", (complex)

Real-World Debugging Examples

1. Password Validation Not Working

// Pattern: ^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$
// Test: "Password1" → Should match
// Test: "password1" → Should NOT match (no uppercase)
// Test: "PASSWORD1" → Should NOT match (no lowercase)
// Test: "Password" → Should NOT match (no digit)
// Debug: Use tester to see which lookahead fails

2. URL Matching Too Much

// Pattern: https?://.+
// Test: "Visit https://example.com and https://test.com"
// Matches: "https://example.com and https://test.com" (greedy!)
// Fix: https?://.+? (lazy) or https?://[^\s]+

3. Capturing Groups Off by One

// Pattern: (\d{3})-(\d{3})-(\d{4})
// Test: "555-123-4567"
// Groups: [0]="555-123-4567", [1]="555", [2]="123", [3]="4567"
// If you add outer group: ((\d{3})-(\d{3})-(\d{4}))
// Groups shift! Use named groups to avoid confusion.

4. Unicode Not Matching

// Pattern: \w+
// Test: "你好" (Chinese) → No match in JS without 'u' flag
// Fix: /\w+/u → Matches (with Unicode property)
// Or: /[\p{L}\p{N}_]+/u for explicit Unicode letters/numbers

Performance & ReDoS Prevention

Catastrophic Backtracking

// DANGEROUS: Exponential backtracking
/(a+)+b/.test("aaaaaaaaaaaaaaaaaaaaac");  // Freezes!

// Input: "a" × 20 + "c"
// Engine tries: (a)(a)(a)... then (aa)(aa)... then (aaa)(aaa)...

Safe Patterns

// Use atomic groups / possessive quantifiers (not in JS, but in PCRE)
// In JS: restructure to avoid nesting quantifiers

// Instead of (a+)+b
// Use a+b (if that's what you mean)
// Or: a{1,20}b (bounded)

Best Practices

Anchor patterns when possible: ^...$
Avoid nested quantifiers: (a+)+
Use specific character classes: [a-z] not .
Bound repetitions: {1,100} not *
Test with long non-matching strings
Profile with console.time() in real code

// Benchmark helper
function benchmark(pattern, input, iterations = 1000) {
  const regex = new RegExp(pattern);
  const start = performance.now();
  for (let i = 0; i < iterations; i++) regex.test(input);
  return performance.now() - start;
}

Programming Language Differences

Feature	JavaScript	Python	Go	Rust	PCRE (PHP)
Named groups	✅ `(?<name>)`	✅ `(?P<name>)`	✅ `(?P<name>)`	✅ `(?P<name>)`	✅ `(?<name>)`
Unicode props	✅ `\p{...}` (u)	✅ `\p{...}`	✅ `\p{...}`	✅ `\p{...}`	✅ `\p{...}`
Lookbehind	✅ `(?<=...)`	✅ `(?<=...)`	❌	✅ `(?<=...)`	✅ `(?<=...)`
Atomic groups	❌	✅ `(?>...)`	❌	✅ `(?>...)`	✅ `(?>...)`
Possessive	❌	✅ `*+` `++`	❌	✅ `*+` `++`	✅ `*+` `++`
Recursion	❌	❌	❌	❌	✅ `(?R)`
Verbose mode	❌	✅ `re.VERBOSE`	❌	✅ `x` flag	✅ `x` flag

FAQ

Why doesn't my regex work in JavaScript but works in Python? Different engines support different features. JS lacks: atomic groups, possessive quantifiers, recursion, verbose mode. Use the tester with target engine in mind.

How do I match a literal backslash? \\\\ in string literal → \\ in regex → matches single \

What's the difference between test() and exec()?

test(): Returns boolean, updates lastIndex (with g flag)
exec(): Returns match array or null, captures groups
match(): String method, returns all matches (with g) or first match array

How do I replace all occurrences?

// With global flag
str.replace(/pattern/g, "replacement")

// With replaceAll (ES2021)
str.replaceAll("literal", "replacement")  // No regex, safer for literals

Can I use regex to parse HTML/XML/JSON? No. Use proper parsers. Regex cannot handle nested structures reliably.

Why is my regex slow on long strings? Likely catastrophic backtracking. Test with the tester's "Performance" mode (shows steps). Restructure pattern.

Does the tester support lookbehind? Yes — (?<=...) and (?<!...) (fixed-width in JS).

What's the maximum regex length? No practical limit. Browser regex engines handle thousands of characters.

Related Tools

Regex Cheatsheet — Quick reference for patterns
String Escape/Unescape — Handle regex in strings
Text Diff Checker — Compare regex outputs
JSON Path Finder — Query JSON instead of regex

References

MDN: Regular Expressions
ECMAScript 2024 RegExp Spec
Regular-Expressions.info
Regex101 — Interactive Tester (PCRE, JS, Python, Go)
OWASP: ReDoS Prevention
Unicode Technical Standard #18
Mastering Regular Expressions (Friedl)

Test regex now → Free Regex Tester — Real-time matching, capture groups, flags, substitution, explanation. Web Worker powered, client-side.

Related Resources

Why Test Regular Expressions?

Regex Fundamentals

Anatomy of a Pattern

/pattern/flags
 ^      ^    ^
 |      |    └─ Flags: g, i, m, s, u, y
 |      └─ Delimiters (JS: /.../, others: implicit)
 └─ The pattern itself

Common Metacharacters

Char	Meaning	Example	Matches
`.`	Any char (except newline)	`c.t`	cat, cot, cxt
`\d`	Digit `[0-9]`	`\d{3}-\d{4}`	555-1234
`\D`	Non-digit	`\D+`	"abc"
`\w`	Word char `[a-zA-Z0-9_]`	`\w+`	"hello123"
`\W`	Non-word char	`\W+`	"!@#"
`\s`	Whitespace	`\s+`	" \t\n"
`\S`	Non-whitespace	`\S+`	"hello"
`^`	Start of string/line	`^Hello`	"Hello world"
`$`	End of string/line	`world$`	"Hello world"
`\b`	Word boundary	`\bcat\b`	"cat" not "catapult"
`\B`	Non-word boundary	`\Bcat\B`	"scatter"

Quantifiers

Quantifier	Meaning	Example
`*`	0 or more	`ab*c` → ac, abc, abbc, abbbc...
`+`	1 or more	`ab+c` → abc, abbc, abbbc...
`?`	0 or 1	`ab?c` → ac, abc
`{n}`	Exactly n	`a{3}` → aaa
`{n,}`	n or more	`a{2,}` → aa, aaa, aaaa...
`{n,m}`	n to m	`a{2,4}` → aa, aaa, aaaa

Greedy vs Lazy:

Greedy (default): .* matches as much as possible
Lazy: .*? matches as little as possible

// HTML tag example
const html = "<div>Hello</div><span>World</span>";

/<div>.*<\/div>/.exec(html);   // Greedy: "<div>Hello</div><span>World</span>"
/<div>.*?<\/div>/.exec(html);  // Lazy:   "<div>Hello</div>"

Character Classes

Pattern	Description
`[abc]`	a, b, or c
`[a-z]`	a through z
`[^abc]`	NOT a, b, or c
`[a-zA-Z0-9]`	Alphanumeric
`[[:alpha:]]`	POSIX alpha (locale-aware)

Capture Groups

Numbered Groups `()`

const regex = /(\d{4})-(\d{2})-(\d{2})/;
const match = regex.exec("2024-01-15");
/*
match[0] = "2024-01-15"  (full match)
match[1] = "2024"        (year)
match[2] = "01"          (month)
match[3] = "15"          (day)
*/

Named Groups `(?<name>)` (ES2018+)

const regex = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/;
const match = regex.exec("2024-01-15");
/*
match.groups = { year: "2024", month: "01", day: "15" }
match[0] = "2024-01-15"
*/

Non-Capturing Groups `(?:)`

// Group for alternation without capturing
/(?:cat|dog|bird)s/  // Matches "cats", "dogs", "birds"
// No extra capture group created

Backreferences `\1`, `\2`, `\k<name>`

// Match repeated words
/\b(\w+)\s+\1\b/.test("hello hello");  // true
/\b(\w+)\s+\1\b/.test("hello world");  // false

// Named backreference
/(?<word>\w+)\s+\k<word>/.test("test test");  // true

Flags (Modifiers)

Flag	Name	Effect
`g`	Global	Find all matches, not just first
`i`	Case-insensitive	`/cat/i` matches "Cat", "CAT", "cat"
`m`	Multiline	`^` and `$` match line boundaries
`s`	DotAll (ES2018)	`.` matches newlines too
`u`	Unicode (ES2015)	Proper Unicode handling, enables `\p{...}`
`y`	Sticky (ES2015)	Match only at `lastIndex` position

// Unicode property escapes (requires 'u' flag)
/\p{Script=Greek}+/u.test("αβγ");     // true
/\p{Emoji}+/u.test("😀🎉");          // true
/\p{General_Category=Letter}/u.test("你好"); // true

How to Test Regex Online (Step by Step)

Open the tester — DevStackIO Regex Tester
Enter pattern — Type regex (with or without /delimiters/)
Set flags — Toggle g, i, m, s, u, y as needed
Enter test string — Paste text, type, or load file
Real-time results — Matches highlight instantly in test string
Inspect groups — Panel shows full match + all capture groups per match
Test substitution — Enter replacement string (supports $1, $&, $<name>)
Explain mode — Get plain-English breakdown of each pattern part
Copy/export — Pattern, matches, or substitution result

Common Patterns Library

Validation

# Email (simplified, RFC 5322 is complex)
^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)+$

# URL
^https?:\/\/(?:www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b(?:[-a-zA-Z0-9()@:%_\+.~#?&\/=]*)$

# IPv4
^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$

# UUID v4
^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$

# Credit card (Luhn check needs code)
^(?:4[0-9]{12}(?:[0-9]{3})?|5[1-5][0-9]{14}|3[47][0-9]{13}|3(?:0[0-5]|[68][0-9])[0-9]{11}|6(?:011|5[0-9]{2})[0-9]{12})$

# Strong password (8+ chars, upper, lower, digit, special)
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[!@#$%^&*]).{8,}$

Extraction

# URLs in text
https?:\/\/(?:www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b(?:[-a-zA-Z0-9()@:%_\+.~#?&\/=]*)

# Email addresses
[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)+

# IP addresses (v4)
\b(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b

# Hashtags
#\w+

# Mentions
@\w+

# Hex colors
#(?:[0-9a-fA-F]{3}){1,2}\b

# Dates (YYYY-MM-DD)
\d{4}-\d{2}-\d{2}

# Version numbers
v?\d+\.\d+\.\d+(?:-[a-zA-Z0-9.]+)?

Transformation

# CamelCase to snake_case
([a-z])([A-Z]) → $1_$2 (then lowercase)

# Snake_case to CamelCase
_([a-z]) → \U$1 (then capitalize first)

# Remove HTML tags
<[^>]*> → (empty)

# Normalize whitespace
\s+ → (space)

# Add quotes to CSV fields
^|,(?=(?:[^"]*"[^"]*")*[^"]*$) → ", (complex)

Real-World Debugging Examples

1. Password Validation Not Working

// Pattern: ^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$
// Test: "Password1" → Should match
// Test: "password1" → Should NOT match (no uppercase)
// Test: "PASSWORD1" → Should NOT match (no lowercase)
// Test: "Password" → Should NOT match (no digit)
// Debug: Use tester to see which lookahead fails

2. URL Matching Too Much

// Pattern: https?://.+
// Test: "Visit https://example.com and https://test.com"
// Matches: "https://example.com and https://test.com" (greedy!)
// Fix: https?://.+? (lazy) or https?://[^\s]+

3. Capturing Groups Off by One

// Pattern: (\d{3})-(\d{3})-(\d{4})
// Test: "555-123-4567"
// Groups: [0]="555-123-4567", [1]="555", [2]="123", [3]="4567"
// If you add outer group: ((\d{3})-(\d{3})-(\d{4}))
// Groups shift! Use named groups to avoid confusion.

4. Unicode Not Matching

// Pattern: \w+
// Test: "你好" (Chinese) → No match in JS without 'u' flag
// Fix: /\w+/u → Matches (with Unicode property)
// Or: /[\p{L}\p{N}_]+/u for explicit Unicode letters/numbers

Performance & ReDoS Prevention

Catastrophic Backtracking

// DANGEROUS: Exponential backtracking
/(a+)+b/.test("aaaaaaaaaaaaaaaaaaaaac");  // Freezes!

// Input: "a" × 20 + "c"
// Engine tries: (a)(a)(a)... then (aa)(aa)... then (aaa)(aaa)...

Safe Patterns

// Use atomic groups / possessive quantifiers (not in JS, but in PCRE)
// In JS: restructure to avoid nesting quantifiers

// Instead of (a+)+b
// Use a+b (if that's what you mean)
// Or: a{1,20}b (bounded)

Best Practices

Anchor patterns when possible: ^...$
Avoid nested quantifiers: (a+)+
Use specific character classes: [a-z] not .
Bound repetitions: {1,100} not *
Test with long non-matching strings
Profile with console.time() in real code

// Benchmark helper
function benchmark(pattern, input, iterations = 1000) {
  const regex = new RegExp(pattern);
  const start = performance.now();
  for (let i = 0; i < iterations; i++) regex.test(input);
  return performance.now() - start;
}

Programming Language Differences

Feature	JavaScript	Python	Go	Rust	PCRE (PHP)
Named groups	✅ `(?<name>)`	✅ `(?P<name>)`	✅ `(?P<name>)`	✅ `(?P<name>)`	✅ `(?<name>)`
Unicode props	✅ `\p{...}` (u)	✅ `\p{...}`	✅ `\p{...}`	✅ `\p{...}`	✅ `\p{...}`
Lookbehind	✅ `(?<=...)`	✅ `(?<=...)`	❌	✅ `(?<=...)`	✅ `(?<=...)`
Atomic groups	❌	✅ `(?>...)`	❌	✅ `(?>...)`	✅ `(?>...)`
Possessive	❌	✅ `*+` `++`	❌	✅ `*+` `++`	✅ `*+` `++`
Recursion	❌	❌	❌	❌	✅ `(?R)`
Verbose mode	❌	✅ `re.VERBOSE`	❌	✅ `x` flag	✅ `x` flag

FAQ

How do I match a literal backslash? \\\\ in string literal → \\ in regex → matches single \

What's the difference between test() and exec()?

test(): Returns boolean, updates lastIndex (with g flag)
exec(): Returns match array or null, captures groups
match(): String method, returns all matches (with g) or first match array

How do I replace all occurrences?

// With global flag
str.replace(/pattern/g, "replacement")

// With replaceAll (ES2021)
str.replaceAll("literal", "replacement")  // No regex, safer for literals

Can I use regex to parse HTML/XML/JSON? No. Use proper parsers. Regex cannot handle nested structures reliably.

Why is my regex slow on long strings? Likely catastrophic backtracking. Test with the tester's "Performance" mode (shows steps). Restructure pattern.

Does the tester support lookbehind? Yes — (?<=...) and (?<!...) (fixed-width in JS).

What's the maximum regex length? No practical limit. Browser regex engines handle thousands of characters.

Related Tools

Regex Cheatsheet — Quick reference for patterns
String Escape/Unescape — Handle regex in strings
Text Diff Checker — Compare regex outputs
JSON Path Finder — Query JSON instead of regex

References

MDN: Regular Expressions
ECMAScript 2024 RegExp Spec
Regular-Expressions.info
Regex101 — Interactive Tester (PCRE, JS, Python, Go)
OWASP: ReDoS Prevention
Unicode Technical Standard #18
Mastering Regular Expressions (Friedl)

Regex Tester with Capture Groups — Debug Patterns in Real-Time

Why Test Regular Expressions?

Regex Fundamentals

Anatomy of a Pattern

Common Metacharacters

Quantifiers

Character Classes

Capture Groups

Numbered Groups ()

Named Groups (?<name>) (ES2018+)

Non-Capturing Groups (?:)

Backreferences \1, \2, \k<name>

Flags (Modifiers)

How to Test Regex Online (Step by Step)

Common Patterns Library

Validation

Extraction

Transformation

Real-World Debugging Examples

1. Password Validation Not Working

2. URL Matching Too Much

3. Capturing Groups Off by One

4. Unicode Not Matching

Performance & ReDoS Prevention

Catastrophic Backtracking

Safe Patterns

Best Practices

Programming Language Differences

FAQ

Related Tools

References

Test regex now → Free Regex Tester — Real-time matching, capture groups, flags, substitution, explanation. Web Worker powered, client-side.

Related Resources

More Blog Posts

Regex Tester with Capture Groups — Debug Patterns in Real-Time

Why Test Regular Expressions?

Regex Fundamentals

Anatomy of a Pattern

Common Metacharacters

Quantifiers

Character Classes

Capture Groups

Numbered Groups ()

Named Groups (?<name>) (ES2018+)

Non-Capturing Groups (?:)

Backreferences \1, \2, \k<name>

Flags (Modifiers)

How to Test Regex Online (Step by Step)

Common Patterns Library

Validation

Extraction

Transformation

Real-World Debugging Examples

1. Password Validation Not Working

2. URL Matching Too Much

3. Capturing Groups Off by One

4. Unicode Not Matching

Performance & ReDoS Prevention

Catastrophic Backtracking

Safe Patterns

Best Practices

Programming Language Differences

FAQ

Related Tools

References

Test regex now → Free Regex Tester — Real-time matching, capture groups, flags, substitution, explanation. Web Worker powered, client-side.

Related Resources

More Blog Posts

Numbered Groups `()`

Named Groups `(?<name>)` (ES2018+)

Non-Capturing Groups `(?:)`

Backreferences `\1`, `\2`, `\k<name>`

Numbered Groups `()`

Named Groups `(?<name>)` (ES2018+)

Non-Capturing Groups `(?:)`

Backreferences `\1`, `\2`, `\k<name>`