What is a regular expression?

A regular expression (regex) is a sequence of characters that defines a search pattern. It's used for pattern matching within strings, allowing you to find, replace, or validate text based on specific criteria.

What do regex flags mean?

Regex flags modify how the pattern matching works. Global (g) finds all matches, case-insensitive (i) ignores letter case, multiline (m) treats beginning and end anchors as working on individual lines, and dotall (s) allows dot to match newline characters.

Regex Tester

Test and debug regular expressions online with live pattern matching. Validate email addresses, phone numbers, URLs, and custom text patterns with syntax highlighting and capture group analysis.

Regular Expression Pattern:

Flags:

Global (g)

Ignore Case (i)

Multiline (m)

Dotall (s)

Test String:

Find & Replace:

Replacement String:

Replacement Results:

Common Regex Patterns:

[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,} Email Address

\b\d{3}-\d{3}-\d{4}\b Phone Number (US)

https?://... URL

^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)... Strong Password

^[0-9]{4}-[0-9]{2}-[0-9]{2}$ Date (YYYY-MM-DD)

^[A-Z]{2}[0-9]{2}... IBAN

About Regular Expressions

What are Regular Expressions?

Regular expressions (regex or regexp) are patterns used to match character combinations in strings. They provide a powerful and flexible way to search, match, and manipulate text data across programming languages and text processing tools.

Common Regex Metacharacters

. (dot): Matches any single character except newline
* (asterisk): Matches zero or more of the preceding character
+ (plus): Matches one or more of the preceding character
? (question mark): Matches zero or one of the preceding character
^ (caret): Matches the start of a line
$ (dollar): Matches the end of a line

Character Classes

[abc]: Matches any character in the set (a, b, or c)
[^abc]: Matches any character NOT in the set
[a-z]: Matches any lowercase letter
[A-Z]: Matches any uppercase letter
[0-9]: Matches any digit
\d: Matches any digit (equivalent to [0-9])
\w: Matches any word character (letters, digits, underscore)
\s: Matches any whitespace character

Quantifiers

{n}: Matches exactly n occurrences
{n,}: Matches n or more occurrences
{n,m}: Matches between n and m occurrences
*: Matches zero or more (equivalent to {0,})
+: Matches one or more (equivalent to {1,})
?: Matches zero or one (equivalent to {0,1})

Regex Flags Explained

Global (g): Find all matches, not just the first one
Case-insensitive (i): Ignore case when matching
Multiline (m): ^ and $ match start/end of each line
Dotall (s): . matches newline characters

Common Questions

How do I escape special characters in regex?

Use a backslash (\) before special characters like . * + ? ^ $ | \ ( ) [ ] { } to match them literally. For example, \. matches a literal period.

What's the difference between * and + quantifiers?

The * quantifier matches zero or more occurrences, while + matches one or more occurrences. Use + when you need at least one match.

How do capture groups work?

Parentheses () create capture groups that save matched portions for later use. For example, (\d{3})-(\d{3})-(\d{4}) captures three groups from a phone number.

Why isn't my regex matching?

Common issues include: forgetting to escape special characters, case sensitivity, incorrect quantifiers, or anchor positions. Use our tester to debug your patterns step by step.