Basic Characters
. - Matches any character except newline\d - Matches any digit (0-9)\D - Matches any non-digit\w - Matches any word character (alphanumeric + underscore)\W - Matches any non-word character\s - Matches any whitespace (space, tab, newline)\S - Matches any non-whitespace character\t - Matches a tab character\n - Matches a newline character\r - Matches a carriage return
Anchors
^ - Start of a string$ - End of a string\b - Word boundary (e.g., matches the boundary between a word and a space)\B - Non-word boundary (e.g., matches the position between two word characters)
Quantifiers
* - 0 or more occurrences (e.g., a* matches "", “a”, “aa”, etc.)+ - 1 or more occurrences (e.g., a+ matches “a”, “aa”, etc.)? - 0 or 1 occurrence (optional) (e.g., a? matches "" or “a”){n} - Exactly n occurrences (e.g., a{3} matches “aaa”){n,} - n or more occurrences (e.g., a{2,} matches “aa”, “aaa”, etc.){n,m} - Between n and m occurrences (e.g., a{2,4} matches “aa”, “aaa”, or “aaaa”)
Groups and Ranges
(abc) - Capturing group, matches exactly “abc”(?:abc) - Non-capturing group, matches “abc” without storing it for backreferences[abc] - Character set, matches “a”, “b”, or “c”[^abc] - Negated character set, matches any character except “a”, “b”, or “c”[a-z] - Character range, matches any lowercase letter from a to z[A-Z] - Matches any uppercase letter from A to Z[0-9] - Matches any digit from 0 to 9[a-zA-Z0-9_] - Matches any alphanumeric character or underscore(a|b) - Alternation, matches either “a” or “b” (OR operator)
Special Characters
\\ - Escape character (e.g., \. to match a period)\n - Newline\t - Tab\r - Carriage return\f - Form feed\v - Vertical tab
Lookahead and Lookbehind
(?=...) - Positive lookahead, matches if ... follows (e.g., \d(?=abc) matches a digit followed by “abc”)(?!...) - Negative lookahead, matches if ... does not follow (e.g., \d(?!abc) matches a digit not followed by “abc”)(?<=...) - Positive lookbehind, matches if ... precedes (e.g., (?<=abc)\d matches a digit preceded by “abc”)(?<!...) - Negative lookbehind, matches if ... does not precede (e.g., (?<!abc)\d matches a digit not preceded by “abc”)
Backreferences
\1, \2, etc. - Matches the same text as the first, second, etc., capturing group (e.g., (\w)\1 matches repeated word characters like “aa” or “bb”)- Named Capturing Groups:
(?<name>...) - Assigns a name to a capturing group (e.g., (?<digit>\d))
- Named Backreference:
\k<name> - Refers to a named capturing group (e.g., \k<digit>)
Useful Shorthands
.* - Matches any character (except newline) 0 or more times\w+ - Matches one or more word characters\d{n} - Matches exactly n digits\s* - Matches zero or more whitespace characters[a-zA-Z]+ - Matches one or more alphabetic characters[A-Za-z0-9_.+-] - Matches characters commonly used in email addresses
Common Regex Patterns
- Email:
\b[\w.%+-]+@[\w.-]+\.[A-Za-z]{2,6}\b
- URL:
http[s]?://(?:[a-zA-Z]|[0-9]|[$-_@.&+]|[!*\(\),]|(?:%[0-9a-fA-F][0-9a-fA-F]))+
- Phone Number (US):
\(?\d{3}\)?[-\s.]?\d{3}[-\s.]?\d{4}
- Date (YYYY-MM-DD):
\b\d{4}-\d{2}-\d{2}\b
- IPv4 Address:
\b(?:\d{1,3}\.){3}\d{1,3}\b
- Postal Code (US):
\b\d{5}(?:-\d{4})?\b
- Hex Color Code:
#(?:[0-9a-fA-F]{3}){1,2}
- Username:
[a-zA-Z0-9_]{3,16} (matches usernames between 3 and 16 characters)
Flags
i - Case insensitive (e.g., /abc/i matches “ABC” as well as “abc”)g - Global search (e.g., /abc/g finds all matches)m - Multi-line mode (^ and $ match start/end of lines)s - Dotall mode (. matches newline as well)u - Unicode mode (e.g., /\u{1F600}/u matches a Unicode emoji)
Tips for Using Regex
- Escape Special Characters: Use
\\ to escape special characters like ., *, +, ?, etc.
- Capturing vs Non-Capturing Groups: Use
(?:...) for non-capturing groups if you don’t need backreferences.
- Testing Regex: Use tools like regex101 or RegExr to test and debug your expressions.
- Use Comments for Complex Regex: In some languages, you can use
x flag to allow comments and whitespace for readability (e.g., / ( [A-Z] \w+ ) /x)
- Break Down Complex Patterns: Break down your pattern into smaller parts to debug step by step.