regex

name	regex
description	Write, explain, test, and debug regular expressions, and build them iteratively against real sample input. Use when users want a regex/pattern to match or extract text, ask why a pattern isn't matching, need to validate emails/URLs/dates/phone numbers, or want to do find-and-replace with capture groups. Triggers on mentions of regex, regular expression, pattern, match, extract, validate, find and replace, 正则, 正则表达式, 匹配, 提取, 替换.
license	Proprietary. LICENSE.txt has complete terms
compatibility	Requires python3. Works on macOS, Linux, and Windows.

Regex

Never hand-wave a regex — test it against real samples before handing it over. A pattern that "looks right" but fails on the user's actual input is worse than no pattern.

Test a pattern against samples

python3 - <<'PY'
import re
pat = r'\b\d{4}-\d{2}-\d{2}\b'
samples = ["due 2026-05-31 ok", "no date", "2026/05/31 wrong sep"]
for s in samples:
    print(repr(s), "->", re.findall(pat, s))
PY

Extract with named groups

python3 - <<'PY'
import re
m = re.search(r'(?P<user>[\w.+-]+)@(?P<domain>[\w.-]+)', "ping [e-mail]")
print(m.groupdict() if m else "no match")
PY

Find-and-replace with backreferences

python3 - <<'PY'
import re
print(re.sub(r'(\d{4})-(\d{2})-(\d{2})', r'\3/\2/\1', "2026-05-31"))  # -> 31/05/2026
PY

Common building blocks

• Anchors: ^ start, $ end, \b word boundary.
• Classes: \d digit, \w word char, \s space; negate with uppercase \D \W \S.
• Quantifiers: * 0+, + 1+, ? 0/1, {m,n}; add ? for lazy (.*?).
• Groups: (...) capture, (?:...) non-capture, (?P<name>...) named.
• Lookaround: (?=...), (?!...), (?<=...), (?<!...).

Guidance

• Always show what it matches AND what it deliberately rejects, using the user's samples.
• Prefer explicit, readable patterns over clever unreadable ones; add re.VERBOSE for complex patterns and comment them.
• Warn about catastrophic backtracking (nested quantifiers like (a+)+) on untrusted input.
• Note the regex flavor if it leaves Python (JS, PCRE, Go/RE2, grep) — escaping and lookbehind support differ.