/compat /regex/regex-features-by-engine

Regex features by engine: JavaScript, Python, PCRE and POSIX ERE

updated 2026-10-20

tool: Regex
versions: Node.js 24, Python 3.13, PCRE2, POSIX ERE
os: any

The core syntax (classes, quantifiers, groups, alternation) is the same everywhere, but the advanced features are not. Python's re writes named groups as (?P<name>…) and has no \p{…}; JavaScript has no atomic groups or possessive quantifiers; POSIX ERE (grep -E, awk, sed -E) has no lookarounds at all.

Tested in JavaScript and Python

bash

cat > cases.tsv <<'EOF'
lookahead	foo(?=bar)	foobar
lookbehind	(?<=\$)\d+	$42
named group (?<n>)	(?<year>\d{4})	2026-10
named group (?P<n>)	(?P<year>\d{4})	2026-10
unicode \p{L}	\p{L}+	café
atomic group	(?>a+)b	aaab
possessive a++	a++b	aaab
backreference	(\w)\1	book
EOF
node -e 'console.log("node", process.version)'; python -c 'import sys; print("python", sys.version.split()[0])'
printf '%-22s %-12s %s\n' FEATURE JAVASCRIPT PYTHON
while IFS=$'\t' read -r name re text; do
  js=$(node -e 'try { const m = new RegExp(process.argv[1], "u").exec(process.argv[2]); console.log(m ? m[0] : "no match") } catch { console.log("error") }' "$re" "$text")
  py=$(python -c 'import re, sys
try:
    m = re.search(sys.argv[1], sys.argv[2]); print(m.group(0) if m else "no match")
except re.error:
    print("error")' "$re" "$text")
  printf '%-22s %-12s %s\n' "$name" "$js" "$py"
done < cases.tsv

Output

node v24.20.0
python 3.13.15
FEATURE                JAVASCRIPT   PYTHON
lookahead              foo          foo
lookbehind             42           42
named group (?<n>)     2026         error
named group (?P<n>)    error        2026
unicode \p{L}          café        error
atomic group           error        aaab
possessive a++         error        aaab
backreference          oo           oo

error means the engine rejects the syntax. Python accepts atomic groups and possessive quantifiers since 3.11.

Full table

Feature	JavaScript	Python re	PCRE2 (grep -P, PHP)	POSIX ERE (grep -E)
lookahead (?= ) (?! )	yes	yes	yes	no
lookbehind (?<= ) (?<! )	yes, any length	yes, fixed length	yes, bounded length	no
named groups	(?<n>…)	(?P<n>…)	both	no
\p{L} Unicode properties	with u or v flag	no (use the regex package)	yes	no
atomic (?>…), possessive a++	no	yes (3.11+)	yes	no
backreferences \1	yes	yes	yes	yes in grep, not in all ERE tools
lazy quantifiers *?	yes	yes	yes	no
\d \w \s	yes	yes (Unicode by default)	yes	use [[:digit:]] [[:alnum:]_] [[:space:]]

Sources

cheat sheets · regexRegex cheat sheet: characters, quantifiers, groups and lookaroundsRegex cheat sheet: character classes, anchors, quantifiers, greedy and lazy matching, groups and backreferences, lookarounds, with tested Python examples.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.

Tested in JavaScript and Python

Full table

Sources

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