Flags

file_re accepts the same flag bitmask as re. Every entry point that takes a flags keyword forwards it to the underlying Rust regex engine after translation. Because file_re uses the Rust regex crate, a few flags behave differently than they do in CPython re and are documented here.

Supported flags

Python flag	Rust handling	Notes
re.IGNORECASE (re.I)	case_insensitive(true)	Full parity with re.
re.MULTILINE (re.M)	multi_line(true)	^ and $ match at line boundaries.
re.DOTALL (re.S)	dot_matches_new_line(true)	. matches \n.
re.VERBOSE (re.X)	ignore_whitespace(true)	Full parity with re.
re.UNICODE (re.U)	Default-on; no-op.	Unicode character classes are always on in Rust’s regex.
re.ASCII (re.A)	unicode(false); emits UserWarning on first use.	Divergent; see below.
re.DEBUG	Ignored; emits UserWarning.	Rust regex has no equivalent debug output.
re.LOCALE (re.L)	Raises ValueError.	No Rust equivalent; re itself discourages this flag.

Flags combine the same way as in re:

import re
from file_re import file_re

match = file_re.search(
    r"^error:.*$",
    "server.log",
    flags=re.IGNORECASE | re.MULTILINE,
)

Rust-vs-Python divergences

re.ASCII

In CPython, re.ASCII scopes \w, \s, \d, and \b to ASCII for the patterns that use them. The Rust regex crate has no equivalent scoping: it only exposes an all-or-nothing unicode(false) switch that turns off Unicode interpretation for the entire pattern. file_re translates re.ASCII to unicode(false), which is broader than what re does.

In practice the results are usually equivalent for ASCII-only input, but patterns that mix ASCII and non-ASCII character classes will not behave identically. To surface this, file_re emits a UserWarning the first time re.ASCII is used in a process:

UserWarning: re.ASCII in file_re disables Unicode character class
matching entirely (Rust regex semantics); this is broader than
Python's re.ASCII.

re.LOCALE

CPython implements re.LOCALE by consulting the current C locale at match time for byte patterns. Rust’s regex has no locale concept and file_re does not attempt to emulate one. Passing re.LOCALE raises ValueError:

import re
from file_re import file_re

file_re.search(r"\w+", "data.txt", flags=re.LOCALE)
# ValueError: re.LOCALE is not supported by file_re.

If you have existing re code that uses re.LOCALE, note that re itself deprecated the flag for str patterns in Python 3.6. The usual migration is to drop the flag and rely on the default Unicode semantics.

re.DEBUG

The Rust regex crate does not have an equivalent of re’s debug print. file_re ignores re.DEBUG and emits a UserWarning so the loss of behavior is visible:

UserWarning: re.DEBUG is not supported by file_re and is ignored.

Inline directives

Inline flags such as (?i), (?m), (?s), and (?x) are handled by the Rust regex crate directly and are always supported regardless of the flags keyword. The underlying compiler uses RegexBuilder, so keyword flags and inline directives compose predictably.