fnmatch Bracket Caret Is Literal, Not Negation (tested)

In regular expressions, [^t] inside brackets means "not t". We measured what Python's fnmatch module does with that exact syntax, and it means something else entirely.

1. A regex-style negated bracket silently matches the thing it was meant to exclude

import fnmatch

files = ["tmp_a.txt", "real_a.txt", "^weird.txt", "tmp_b.txt"]
pattern = "[^t]*.txt"  # intended: "does NOT start with t"
print(fnmatch.filter(files, pattern))

['tmp_a.txt', '^weird.txt', 'tmp_b.txt']

Measured: the pattern was meant to keep everything except files starting with t, but it kept exactly the two files starting with t plus a literal edge case -- the opposite of the intent -- with no exception or warning.

2. Why: fnmatch brackets only treat '!' as negation; '^' is just a literal character

import fnmatch

print(fnmatch.translate("[^t]*.txt"))
print(fnmatch.translate("[!t]*.txt"))

(?s:[\^t].*\.txt)\Z
(?s:[^t].*\.txt)\Z

Measured: fnmatch.translate() shows exactly what happens -- [^t] compiles to the regex char class [\^t], matching a literal ^ or t. Only [!t] (fnmatch's own negation syntax) compiles to the real negated regex class [^t]. The ^ you typed never becomes a negation operator; it becomes one more character to match.

3. Fix: use '!' for negation in fnmatch/glob brackets, never '^'

import fnmatch

files = ["tmp_a.txt", "real_a.txt", "^weird.txt", "tmp_b.txt"]
print(fnmatch.filter(files, "[!t]*.txt"))

['real_a.txt', '^weird.txt']

Measured: swapping ^ for ! gives the correct result -- every file not starting with t, including the literal ^weird.txt which was never a t-file to begin with. This same rule applies to glob.glob() and pathlib.Path.glob(), since both build their matching on fnmatch.translate() internally.

Written with AI assistance. Measured on 2026-10-04 with Python 3.13.5's built-in fnmatch module: ran fnmatch.filter() with a bracket pattern using regex-style '^' negation, then compared against fnmatch's actual negation syntax '!'. PlainHeaders is our extension; Python's fnmatch module is built in.