regex
^Notes
Useful for line-based matching with multiline mode or full-string validation when combined with `$`.
Regex Cheat Sheet
Core regular expression syntax, classes, quantifiers, groups, lookarounds, flags, and performance techniques.
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Anchors and Boundaries
Match start/end positions and boundaries rather than literal characters.
regex
$Notes
Use with `^` for full-line or full-string matching.
regex
\bNotes
Common for matching whole words like `\berror\b`.
regex
\BNotes
Useful for matching substrings that must stay inside a larger token.
regex
\ANotes
Common in PCRE, Ruby, and Python-style engines; safer than `^` when multiline mode is enabled.
regex
\ZNotes
Often used with `\A` for strict whole-string matches in PCRE-style engines.
Character Classes
Match one character from a set or shorthand class.
regex
\dNotes
Equivalent to `[0-9]` in many engines.
regex
\DNotes
Inverse of `\d`.
regex
\wNotes
Usually letters, digits, and underscore; exact behavior varies with Unicode settings.
regex
\WNotes
Inverse of `\w`.
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\sNotes
Often includes spaces, tabs, newlines, and other space-like code points.
regex
\SNotes
Inverse of `\s`.
regex
[a-z]Notes
A range inside square brackets matches one character between the endpoints.
regex
[^0-9]Notes
A leading `^` inside `[]` negates the class.
regex
[-_]Notes
Put `-` first or last, or escape it, to avoid range behavior.
regex
\p{L}Notes
Available in engines with Unicode property support.
regex
\p{N}Notes
Useful for international numeric content beyond ASCII digits.
Quantifiers
Control how many times the previous token may repeat.
regex
ab*Notes
Matches `a`, `ab`, `abb`, and so on.
regex
ab+Notes
Requires one or more `b` characters after `a`.
regex
colou?rNotes
Matches both `color` and `colour`.
regex
\d{4}Notes
A common pattern for year-like numeric fields.
Range repetition
Match between a minimum and maximum number of repetitions.
regexANYquantifierscounts
regex
\w{3,10}Notes
Useful for usernames and token length checks.
regex
\d{2,}Notes
Useful when only the lower bound matters.
regex
<.*?>Notes
Common for tag-like extraction where greedy matching would overrun.
regex
".+?"Notes
Useful for quoted strings without crossing too far.
regex
\d++Notes
Supported in PCRE/Java-style engines; useful to reduce catastrophic backtracking.
Groups and Alternation
Capture, group, and branch patterns.
regex
(\d{4})-(\d{2})-(\d{2})Notes
Groups can be referenced later by number.
regex
(?:cat|dog)Notes
Prefer when grouping is needed but capture data is not.
regex
error|warning|infoNotes
Equivalent to logical OR across branches.
regex
^(?:https?|ftp)://([^/\s]+)Notes
Captures the host while allowing several schemes.
regex
^(?:\+1[-.\s]?)?\d{3}[-.\s]?\d{3}[-.\s]?\d{4}$Notes
Common for optional prefixes in validators.
regex
(?|(Mr|Ms)\.?\s+(\w+)|(Dr)\.?\s+(\w+))Notes
PCRE feature useful when alternate branches should expose the same capture numbers.
Backreferences
Reuse earlier captures within the same regex.
regex
\b(\w+)\s+\1\bNotes
Useful for finding duplicated words like `the the`.
regex
(["\']).*?\1Notes
Captures either single or double quote and requires the same closer.
regex
<([A-Za-z][A-Za-z0-9]*)\b[^>]*>.*?</\1>Notes
Works for simple tag-like text, though regex is not a full HTML parser.
regex
(?<word>\w+)\s+\k<word>Notes
Named syntax varies by engine but improves readability.
Lookarounds
Assert context without consuming characters.
regex
\w+(?=:)Notes
Matches the word before a colon without including the colon.
regex
foo(?!bar)Notes
Matches `foo` not followed by `bar`.
regex
(?<=\$)\d+(?:\.\d{2})?Notes
Matches a currency amount after a dollar sign.
regex
(?<!\$)\b\d+\bNotes
Matches bare numbers not immediately preceded by `$`.
regex
(?:(?!END).)+Notes
Useful when you need to stop before a sentinel substring.
Flags and Modes
Inline and external flags that modify matching behavior.
regex
(?i)errorNotes
Equivalent to using an external ignore-case flag in many tools.
regex
(?m)^ERROR:Notes
Useful for log parsing.
regex
(?s)<script.*?</script>Notes
Useful when matching across line breaks.
regex
(?x) ^ \d{4} - \d{2} - \d{2} $Notes
Improves readability for long patterns.
regex
(?u)\b\p{L}+\bNotes
Exact syntax varies by engine, but the idea is to use Unicode classes safely.
regex
(?i:api)-v[0-9]+Notes
Applies case-insensitive matching just to the group.
Escaping and Literals
Match literal symbols safely.
regex
\.Notes
Dot is special outside a character class, so escape it when matching a literal `.`.
regex
\\Notes
Backslashes themselves must usually be escaped.
regex
[\^\$\.\|\?\*\+\(\)\{\}]Notes
A class can list literal metacharacters more conveniently.
regex
\Q[a-z]+(foo)?\ENotes
Supported in PCRE-style engines to avoid escaping many characters one by one.
Performance and Backtracking
Patterns that avoid pathological backtracking.
regex
(?>\d{4}-\d{2}-\d{2})Notes
Supported in PCRE/Java-style engines; useful in complex validators.
Possessive class quantifier
Consume as much as possible without backtracking.
regexANYperformancepossessive
regex
[A-Z]++Notes
Can reduce backtracking in ambiguous patterns.
regex
(.*a)*Notes
Nested unlimited quantifiers can explode on some inputs. Prefer more precise tokens and anchors.
Constrained tag content
Use explicit stop characters instead of dot-star where possible.
regexANYperformanceprecision
regex
<title>[^<]*</title>Notes
Usually faster and more predictable than `<title>.*?</title>`.
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