Regex & Debugging Tools

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🔍 Regex & Debugging

Test, replace, and master regular expressions

🧪 Regex Tester

Regular Expression
Flags:
Test String
Quick Reference — Common Patterns
.Any character (except newline)
\dDigit [0-9]
\DNon-digit
\wWord char [a-zA-Z0-9_]
\WNon-word char
\sWhitespace
\SNon-whitespace
[abc]Character set
[^abc]Negated set
[a-z]Range
^Start of string/line
$End of string/line
\bWord boundary
*0 or more
+1 or more
?0 or 1 (optional)
{n}Exactly n times
{n,m}Between n and m times
(abc)Capture group
(?:abc)Non-capturing group
a|bAlternation (or)
(?=abc)Lookahead
(?!abc)Negative lookahead
(?<=abc)Lookbehind
Usage Guide

What it does

Tests a regular expression pattern against a string and highlights all matches. Shows match count, capture groups, and match indices.

How to use

  • Enter your regex pattern (without delimiters)
  • Select flags: g (global), i (case-insensitive), m (multiline), s (dotAll)
  • Paste or type the test string
  • Click Test Regex to see highlighted matches and group details

Sample Input

Pattern: (\w+)@(\w+)\.(\w+)
Flags:   g, i
Text:    Contact us at info@example.com or support@test.org

Sample Output

2 matches found

Match 1: "info@example.com" (index 17-33)
  Group 1: "info"
  Group 2: "example"
  Group 3: "com"

Match 2: "support@test.org" (index 37-53)
  Group 1: "support"
  Group 2: "test"
  Group 3: "org"

Real-World Use Cases

  • Validating email addresses, phone numbers, URLs
  • Extracting data from log files or structured text
  • Building search-and-replace patterns for code refactoring
  • Testing patterns before using in grep, sed, or programming languages

🔄 Regex Replace

Regular Expression
Flags:
Replacement String Source Text
Result will appear here…
Usage Guide

What it does

Performs regex-based search and replace on text. Supports capture group references ($1, $2, etc.) in the replacement string.

How to use

  • Enter the regex pattern to search for
  • Enter the replacement string (use $1, $2 for captured groups, $& for full match)
  • Paste the source text
  • Click Replace

Sample Input

Pattern:     (\d{4})-(\d{2})-(\d{2})
Replacement: $2/$3/$1
Text:        Event date: 2024-12-25

Sample Output

Event date: 12/25/2024

Replacement Tokens

  • $1, $2, $3... — Captured group by number
  • $& — Entire matched string
  • $` — Text before the match
  • $' — Text after the match
  • $$ — Literal dollar sign

Real-World Use Cases

  • Reformatting dates, phone numbers, or IDs
  • Renaming variables in code (e.g., camelCase → snake_case)
  • Cleaning up log files or CSV data
  • Batch text transformations

🛡️ Regex Escape / Unescape

Result will appear here…
Usage Guide

What it does

Escape: Adds backslashes before all regex special characters so the string is treated as a literal match.
Unescape: Removes unnecessary backslashes from a regex pattern to reveal the plain text.

Characters Escaped

. * + ? ^ $ { } [ ] ( ) | \ /

How to use

  • Select Escape or Unescape mode
  • Enter your string
  • Click Convert

Sample Input (Escape)

Price is $9.99 (USD)

Sample Output

Price is \$9\.99 \(USD\)

Sample Input (Unescape)

Price is \$9\.99 \(USD\)

Sample Output

Price is $9.99 (USD)

Real-World Use Cases

  • Building dynamic regex patterns from user input
  • Escaping file paths or URLs for regex matching
  • Making literal strings safe for RegExp() constructor
  • Debugging why a regex isn’t matching — unescape to see what it actually searches

📚 Regex Cheatsheet

Character Classes

PatternDescriptionExample
.Any character except newlinea.c → abc, a1c
\dDigit [0-9]\d+ → 123, 42
\DNon-digit [^0-9]\D+ → abc, !@#
\wWord character [a-zA-Z0-9_]\w+ → hello_42
\WNon-word character\W+ → @#!, …
\sWhitespace (space, tab, newline)a\sb → a b
\SNon-whitespace\S+ → hello
[abc]Character set — matches a, b, or c[aeiou] → vowels
[^abc]Negated set — anything except a, b, c[^0-9] → non-digits
[a-z]Range — lowercase letters[a-zA-Z] → letters

Quantifiers

PatternDescriptionExample
*0 or more (greedy)ab*c → ac, abc, abbc
+1 or more (greedy)ab+c → abc, abbc
?0 or 1 (optional)colou?r → color, colour
{n}Exactly n times\d{3} → 123
{n,}n or more times\d{2,} → 12, 123, 1234
{n,m}Between n and m times\d{2,4} → 12, 123, 1234
*?0 or more (lazy/non-greedy)<.*?> → first tag only
+?1 or more (lazy/non-greedy)“.+?” → first quoted
??0 or 1 (lazy)colou??r → prefers color

Anchors

PatternDescriptionExample
^Start of string (or line with m flag)^Hello → starts with Hello
$End of string (or line with m flag)world$ → ends with world
\bWord boundary\bword\b → whole word
\BNon-word boundary\Bword → mid-word only

Groups & References

PatternDescriptionExample
(abc)Capturing group(ha)+ → haha
(?:abc)Non-capturing group(?:ha)+ → haha
(?<name>abc)Named capturing group(?<year>\d{4})
\1Backreference to group 1(\w+)\s\1 → word word
\k<name>Named backreference\k<year>
a|bAlternation (or)cat|dog → cat or dog

Lookaheads & Lookbehinds

PatternDescriptionExample
(?=abc)Positive lookahead\d(?=px) → 3 in 3px
(?!abc)Negative lookahead\d(?!px) → 3 not before px
(?<=abc)Positive lookbehind(?<=\$)\d+ → 99 in $99
(?<!abc)Negative lookbehind(?<!\$)\d+ → not after $

Flags

FlagDescriptionExample
gGlobal — find all matches, not just first/a/g → all a’s
iCase-insensitive matching/hello/i → Hello, HELLO
mMultiline — ^ and $ match line boundaries/^line/m → each line
sDotAll — . also matches newline/a.b/s → a\nb
uUnicode — full Unicode matching/\u{1F600}/u → 😀
ySticky — match at exact positionlastIndex anchored
Usage Guide

What it does

An interactive, searchable reference of regex syntax organized by category. Filter by keyword to quickly find the pattern you need.

How to use

  • Type in the search box to filter patterns (searches across pattern, description, and examples)
  • Browse categories: Character Classes, Quantifiers, Anchors, Groups, Lookaheads, Flags

Pro Tips

  • Use the Regex Tester tab to try out any pattern from this cheatsheet
  • Combine patterns — e.g., \b\d{3}-\d{4}\b matches phone numbers like 555-1234
  • Start simple and build up — test each piece of your regex individually