LLM Best Practices

Python Regex (re module) Cheatsheet

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Overview

Python’s re module uses Perl-style syntax but is not PCRE-compatible. Shared syntax (classes, quantifiers, lookarounds) and ready-made patterns are in regex-patterns. Compile patterns you reuse: re.compile() makes intent explicit and avoids repeated cache lookups.

Functions

Use fullmatch to validate and finditer to extract.

FunctionReturnsUse when
re.search(p, s)Match or NoneFind the first match anywhere.
re.match(p, s)Match or NoneThe pattern must match at the start.
re.fullmatch(p, s)Match or NoneThe whole string must match (validation).
re.findall(p, s)ListAll matches as strings, or tuples with several groups.
re.finditer(p, s)Iterator of matchesMatches with positions and groups.
re.sub(p, repl, s)StringReplace; repl may be a string with \g<name> or a function.
re.split(p, s)ListSplit; capturing groups appear in the result.
re.escape(s)StringEscape text for use as a literal in a pattern.

Flags

Pass re.I | re.M, or write them inline: (?i) for the whole pattern, (?i:...) for a group.

FlagInlineEffect
re.IGNORECASE(?i)Case-insensitive.
re.MULTILINE(?m)^ and $ match at every line.
re.DOTALL(?s). also matches newline.
re.VERBOSE(?x)Ignore whitespace and # comments in the pattern.
re.ASCII(?a)\w, \d, \s, \b match ASCII only (Unicode is the default for str).

Python-specific syntax

Use the Python spellings; PCRE and JavaScript syntax differs.

SyntaxMeaning
(?P<name>...), (?P=name)Named group, named backreference. JavaScript spells these (?<name>...) and \k<name>.
\g<name> or \g<1> in replGroup reference in a replacement string.
a*+, (?>...)Possessive quantifier, atomic group (3.11+).
(?<=ab)cLookbehind must be fixed-width; the third-party regex module allows variable width.
import re
 
m = re.search(r"(?P<y>\d{4})-(?P<m>\d{2})-(?P<d>\d{2})", "on 2026-05-14")
m.groupdict()                                       # {'y': '2026', 'm': '05', 'd': '14'}
re.sub(r"\d+", lambda m: str(int(m[0]) * 2), "a1 b2")   # 'a2 b4'
re.sub(r"(?P<last>\w+), (?P<first>\w+)", r"\g<first> \g<last>", "Smith, John")

Gotchas

Avoid these; each returns a surprising match or hangs.

GotchaFix
re.match anchors only the start.Use re.fullmatch for validation.
$ also matches before a trailing newline: match(r"a$", "a\n") succeeds.Use fullmatch or \Z.
\d and \w match non-ASCII digits and letters in str patterns.Use [0-9] or re.ASCII.
findall returns strings, one group’s text, or tuples depending on group count.Use finditer and read .group().
re.split(r"(\s+)", s) keeps the separators.Use (?:\s+) to drop them.
User text interpolated into a pattern changes its meaning.Wrap it in re.escape().
Nested quantifiers like (a+)+ backtrack catastrophically on near-matches.Rewrite the pattern; cap input length.
A bytes pattern on str input raises TypeError.Keep both the same type.

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