About

Regular expressions in Python's re module use a syntax that diverges from other flavors in subtle but critical ways. A pattern that works in JavaScript or PCRE may silently fail in Python due to differences in named group syntax ((?P<name>...) vs (?<name>...)), flag handling, or Unicode defaults. Mismatched regex between environments is a common source of data extraction bugs that pass unit tests but fail on edge-case production input. This tool interprets Python-flavored regex patterns and maps them to equivalent execution, showing all matches, captured groups, and positional data against your test string in real time.

The tester supports Python's standard flags: re.IGNORECASE, re.MULTILINE, re.DOTALL, and re.VERBOSE. It translates Python-specific constructs like (?P<name>...) named groups and (?P=name) backreferences into executable form. Note: this tool approximates Python regex behavior using a translation layer. Lookbehind width restrictions and certain Unicode property escapes (\p{...}) may differ from CPython's re module. For production validation, always confirm against the actual Python interpreter.

Formulas

Python regex matching follows a deterministic process. The re module compiles a pattern string into an internal automaton, then executes it against the target string.

re.findall(pattern, string, flags) → [match₀, match₁, ..., match_n]

Each match_i contains positional data: start (inclusive index), end (exclusive index), and the matched substring string[start:end]. For patterns with groups, each match returns a tuple of captured groups rather than the full match.

match.span() = (start, end) where end − start = len(match.group(0))

Python-to-JS syntax translation follows these rules: (?P<name>...) → (?<name>...) (named group), (?P=name) → \k<name> (backreference). The re.VERBOSE flag strips unescaped whitespace and # comments before compilation.

Reference Data

Python Flag	Short Form	Inline	Effect
re.IGNORECASE	re.I	(?i)	Case-insensitive matching for ASCII and Unicode
re.MULTILINE	re.M	(?m)	^ and $ match at line boundaries
re.DOTALL	re.S	(?s)	. matches any character including \n
re.VERBOSE	re.X	(?x)	Allows whitespace and comments in pattern
re.ASCII	re.A	(?a)	ASCII-only matching for \w, \b, \d, \s
re.UNICODE	re.U	(?u)	Unicode matching (default in Python 3)
Common Python Regex Constructs
(?P<name>...)		Named capturing group (Python syntax)
(?P=name)		Backreference to named group
(?:...)		Non-capturing group
(?=...)		Positive lookahead
(?!...)		Negative lookahead
(?<=...)		Positive lookbehind (fixed-width in Python)
(?<!...)		Negative lookbehind (fixed-width in Python)
(?(id)yes\|no)		Conditional pattern (if group matched)
\A		Start of string (not affected by MULTILINE)
\Z		End of string (not affected by MULTILINE)
\b		Word boundary
\B		Non-word boundary
\d / \D		Digit / Non-digit (Unicode-aware by default)
\w / \W		Word char / Non-word char (Unicode-aware)
\s / \S		Whitespace / Non-whitespace
{m,n}		Between m and n repetitions
{m,n}?		Non-greedy (lazy) repetition
[^...]		Negated character class

Frequently Asked Questions

Both define named capturing groups. Python uses (?P...) as its canonical syntax (inherited from the original regex module by Fredrik Lundh). JavaScript, .NET, and PCRE use (?...). Python 3.6+ also accepts the shorter form, but (?P...) remains the standard in Python codebases. This tool translates the Python form to the JS-compatible form for execution.

Python's re module supports fixed-width lookbehinds only - the lookbehind content must match a string of fixed length. However, Python's alternative regex module (regex, not re) supports variable-width lookbehinds. This tool uses the JavaScript regex engine, which in modern browsers supports variable-width lookbehinds. If your pattern uses a fixed-width lookbehind, it should work identically. Patterns relying on Python-specific regex module extensions may differ.

When re.VERBOSE is active, unescaped whitespace in the pattern is ignored, and # starts a comment that runs to end of line. This allows multi-line, documented patterns. The tool preprocesses verbose patterns by stripping comments (from unescaped # to newline) and removing unescaped whitespace before compilation. Whitespace inside character classes [...] is preserved, as per Python's behavior.

Patterns like (a+)+ against long non-matching strings cause exponential backtracking. This tool enforces a 2-second execution timeout. If matching exceeds this limit, execution is aborted and an error is displayed. To fix such patterns, use atomic groups (not available in Python's re module), possessive quantifiers, or restructure the alternation. The most common fix is replacing (a+)+ with a+ or using a non-capturing group (?:a+)+.

In Python 3, \d matches any Unicode decimal digit by default (e.g., Arabic-Indic digits ٠-٩). With re.ASCII flag, \d matches only [0-9]. JavaScript's \d always matches [0-9] only, unless the Unicode flag (u) is used with Unicode property escapes. This tool applies the re.ASCII behavior by default. Enable the re.UNICODE checkbox if you need broader matching, though full Unicode digit equivalence may vary.

Yes. Enter a replacement string in the substitution field. Python replacement syntax uses \1 or \g for backreferences. The tool converts \g to $ and numeric backreferences \1 to $1 for JS execution. The substitution result is shown in real time below the match display.

Positions should be identical for ASCII strings. For strings containing multi-byte Unicode characters (emoji, CJK), Python counts codepoints while JavaScript's string indexing counts UTF-16 code units. A single emoji (e.g., 🎉, U+1F389) is 1 codepoint in Python but 2 code units in JavaScript. Match content will be correct, but reported indices may differ by the number of surrogate pairs preceding the match.