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re module

Monty’s re module is backed by the Rust fancy-regex crate, not CPython’s regex engine. Most patterns behave identically; the engines differ in syntax extensions and error reporting.

Module functions

Implemented: compile, search, match, fullmatch, findall, sub, split, finditer, escape.

Not implemented: subn, purge, template. The pre-compiled re._compile internal is not exposed.

The module-level functions accept the same positional-or-keyword arguments as CPython (including compiled re.Pattern objects as the first argument, with re.compile(p) is p and the cannot process flags argument with a compiled pattern ValueError), and their signature/type error messages match CPython’s, with these divergences:

  • bytes patterns and subjects are not supported (Monty has no bytes matching). A bytes subject raises CPython’s mixed-types message (cannot use a string pattern on a bytes-like object); a bytes pattern raises first argument must be string or compiled pattern, whereas CPython supports bytes patterns.
  • A non-str repl for re.sub raises callable replacement is not yet supported in re.sub(); CPython raises decoding to str: need a bytes-like object, int found-style errors for non-callable non-strings. Like CPython, the check runs even when a negative count means zero substitutions.
  • re.sub replacement templates are not validated: CPython parses the template eagerly (even with zero matches or a negative count) and raises PatternError for an invalid group reference (\2 with one group: invalid group reference 2 at position 1) or an unknown escape (\q: bad escape \q at position 0). Monty expands references to missing groups as the empty string and passes unknown escapes through literally.
  • A negative or > 0xFFFF integer flags value raises TypeError: flags must be a non-negative integer; CPython accepts larger int-sized values and handles negative values according to the resulting flag bits.
  • Positional count / maxsplit for re.sub / re.split do not emit CPython 3.13+‘s DeprecationWarning (Monty has no warnings machinery).

Flags

Supported: NOFLAG, IGNORECASE / I, MULTILINE / M, DOTALL / S, ASCII / A.

Not implemented: VERBOSE / X, LOCALE / L, DEBUG, UNICODE / U (Unicode is always on). Unknown flag bits within the accepted u16 range are silently accepted; bits above that range are rejected by the integer range check.

re.Pattern objects

Attributes: pattern, flags. Methods: search, match, fullmatch, findall, sub, split, finditer.

Not implemented: subn, groups (count), groupindex (named-group mapping), scanner. The pos / endpos arguments accepted by Pattern.search(string, pos, endpos) etc. in CPython are not supported.

A non-str subject passed to a Pattern method raises expected string, not {type} rather than CPython’s expected string or bytes-like object, got '{type}'. The module-level functions match CPython’s wording.

re.Match objects

Attributes: re, string. Methods: group, groups, groupdict, start, end, span.

Not implemented: lastindex, lastgroup, expand, pos, endpos, regs. Indexing (m[0], m["name"]) is not supported; use .group().

re.PatternError / re.error

Raised for invalid regex patterns. Unlike CPython, pattern, pos, lineno, and colno attributes are not populated: fancy-regex’s error representation does not carry them.

Engine-level differences

  • Unsupported regex features (some Unicode property escapes, some CPython-specific extensions) raise re.PatternError at compile time.
  • Backreference syntax \10 and higher is not recognized; only \1–\9.
  • Error messages for invalid patterns come from fancy-regex and do not match CPython’s wording.
  • Patterns whose compiled form is very large, e.g. huge counted repeats like a{5000000}, raise re.PatternError (fancy-regex/regex enforce a compiled-size limit), whereas CPython compiles them.