Regex cheat sheet: characters, quantifiers, groups and lookarounds

updated 2026-10-20

tool: Regex
versions: PCRE2, Python re, JavaScript
os: any

The regular expression syntax shared by PCRE, Python, JavaScript, Java and .NET. Differences between engines are listed in regex features by engine.

Characters and classes

Pattern	Matches
.	any character except a newline
\d \w \s	digit, word character [A-Za-z0-9_], whitespace
\D \W \S	the opposites
[abc] [a-z] [^0-9]	one of, a range, none of
\. \* \\	a literal dot, star, backslash

Anchors

Pattern	Matches
^ $	start and end of the string (of each line in multiline mode)
\b \B	word boundary, not a word boundary
\A \z	start and end of the string only (Python uses \Z for the end)

Quantifiers

Pattern	Meaning
* + ?	0 or more, 1 or more, 0 or 1
{3} {2,} {2,5}	exactly 3, at least 2, 2 to 5
*? +? ??	lazy versions: as few as possible

Groups and lookarounds

Pattern	Meaning
(abc)	capturing group, \1 refers back to it
(?:abc)	non-capturing group
(?P<name>abc) / (?<name>abc)	named group (Python / PCRE, JavaScript, .NET)
a\|b	alternation
(?=x) (?!x)	followed by x, not followed by x
(?<=x) (?<!x)	preceded by x, not preceded by x

Common patterns, tested

bash

python - <<'EOF'
import re
tests = [
    (r"\b\d{4}-\d{2}-\d{2}\b", "released 2026-10-20, patched 2026-11-02"),
    (r"(?P<key>\w+)=(?P<value>[^&]*)", "q=regex&lang=en&page="),
    (r"#[0-9a-fA-F]{6}\b", "colors: #0e7c86 #FFF #a1b2c3"),
    (r"(?<=\$)\d+(?:\.\d{2})?", "total $42.50, tax $3"),
    (r"^\s*#.*$", "x = 1\n  # comment\ny = 2"),
    (r"\b(\w+)\s+\1\b", "this is is a test test"),
]
for pattern, text in tests:
    flags = re.M if pattern.startswith("^") else 0
    print(f"{pattern:32} -> {[m.group(0) for m in re.finditer(pattern, text, flags)]}")
EOF

Output

\b\d{4}-\d{2}-\d{2}\b            -> ['2026-10-20', '2026-11-02']
(?P<key>\w+)=(?P<value>[^&]*)    -> ['q=regex', 'lang=en', 'page=']
#[0-9a-fA-F]{6}\b                -> ['#0e7c86', '#a1b2c3']
(?<=\$)\d+(?:\.\d{2})?           -> ['42.50', '3']
^\s*#.*$                         -> ['  # comment']
\b(\w+)\s+\1\b                   -> ['is is', 'test test']

Sources

compat · regexRegex features by engine: JavaScript, Python, PCRE and POSIX EREWhich regex features work where: lookbehind, named groups, \p{L}, atomic groups and possessive quantifiers in JavaScript, Python re, PCRE and POSIX ERE, tested.flags · regexJavaScript regex flags: g, i, m, s, u, v, y and dWhat each JavaScript RegExp flag does: g finds all matches, i ignores case, m works per line, s lets . match newlines, u and v add Unicode, plus y and d.glossary · regexGreedy vs lazy quantifiers in regexA greedy quantifier such as .* matches as much as it can and backtracks only when needed; a lazy one such as .*? matches as little as possible. Examples.

Characters and classes

Anchors

Quantifiers

Groups and lookarounds

Common patterns, tested

Sources

Related pages