text.slugify

Turn a title into a lowercase ASCII URL slug, transliterating common accented Latin letters.

1.0.0 · published 2026-10-03 by charlie · Anterra

Pinned by 21 tests, run in TypeScript, Python and Rust.

What it does

The output alphabet is exactly a-z, 0-9 and the hyphen. Runs of anything else collapse to one hyphen, and there is never a leading or trailing one.

Transliteration covers one closed, nameable set: every letter of the Latin-1 supplement (U+00C0 to U+00FF) plus the seven letters Windows-1252 adds (OE, oe, S-caron, s-caron, Z-caron, z-caron, Y-diaeresis). Diacritics are dropped rather than expanded, so a-umlaut becomes a, not ae - the convention every URL slug in the wild follows. The exceptions are the letters that are not accented vowels at all: sharp s becomes ss, ae-ligature becomes ae, oe-ligature becomes oe, thorn becomes th.

For example

slugify(Hello World) → hello-world an ordinary title
slugify(already-a-slug) → already-a-slug something already a slug is left alone
slugify( --Héllo, Wörld!-- ) → hello-world punctuation, repeated spaces and stray hyphens all collapse to one hyphen

The function

The same function in TypeScript, Python and Rust, pinned by the same tests. Pick your language; the choice follows you around the registry.

def slugify(value: str) -> str

value	string	the title or label to slugify
returns	string	lowercase a-z, 0-9 and single hyphens; possibly empty

Your code names it in one line, in the file that uses it

from fune.text.slugify import slugify  # text.slugify@^1

impl/python.py · 77 lines · open · raw

from typing import Dict

# The transliteration table: every letter of the Latin-1 supplement
# (U+00C0-U+00FF) plus the seven letters Windows-1252 adds.
#
# Spelled out character by character rather than left to unicodedata, because
# that is the only way three languages with three different Unicode stacks can
# be pinned to the same output by one vector.
#
# Diacritics are dropped rather than expanded, so "a-umlaut" is "a" and not
# "ae": that is the convention every URL slug in the wild follows. The
# exceptions are the letters that are not accented vowels at all - "sharp s" is
# genuinely two letters, and the ligatures and thorn are letters in their own right.
TRANSLITERATIONS: Dict[str, str] = {
    "À": "a", "Á": "a", "Â": "a", "Ã": "a", "Ä": "a",
    "Å": "a", "Æ": "ae", "Ç": "c", "È": "e", "É": "e",
    "Ê": "e", "Ë": "e", "Ì": "i", "Í": "i", "Î": "i",
    "Ï": "i", "Ð": "d", "Ñ": "n", "Ò": "o", "Ó": "o",
    "Ô": "o", "Õ": "o", "Ö": "o", "Ø": "o", "Ù": "u",
    "Ú": "u", "Û": "u", "Ü": "u", "Ý": "y", "Þ": "th",
    "ß": "ss",
    "à": "a", "á": "a", "â": "a", "ã": "a", "ä": "a",
    "å": "a", "æ": "ae", "ç": "c", "è": "e", "é": "e",
    "ê": "e", "ë": "e", "ì": "i", "í": "i", "î": "i",
    "ï": "i", "ð": "d", "ñ": "n", "ò": "o", "ó": "o",
    "ô": "o", "õ": "o", "ö": "o", "ø": "o", "ù": "u",
    "ú": "u", "û": "u", "ü": "u", "ý": "y", "þ": "th",
    "ÿ": "y",
    "Œ": "oe", "œ": "oe", "Š": "s", "š": "s",
    "Ž": "z", "ž": "z", "Ÿ": "y",
}


def slugify(value: str) -> str:
    """A URL-safe slug: lowercase ASCII letters and digits, single hyphens
    between them, none at either end.

    Anything outside the transliteration table is dropped rather than guessed,
    so a title written entirely in another script slugifies to "". That empty
    string is returned, not raised: the caller knows what its fallback is (an
    id, a date, a hash) and this function does not.
    """
    if not isinstance(value, str):
        raise TypeError("slugify needs a string, received %r" % (value,))

    out = []
    length = 0
    # A pending separator rather than a trailing hyphen plus a strip: it
    # collapses runs and drops the leading and trailing ones in one pass.
    pending_separator = False

    for ch in value:
        code = ord(ch)
        if 0x61 <= code <= 0x7A or 0x30 <= code <= 0x39:
            piece = ch
        elif 0x41 <= code <= 0x5A:
            piece = chr(code + 32)
        else:
            piece = TRANSLITERATIONS.get(ch, "")

        if piece == "":
            pending_separator = length > 0
            continue
        if pending_separator:
            out.append("-")
            length += 1
            pending_separator = False
        out.append(piece)
        length += len(piece)

    return "".join(out)


def slugify_or(value: str, fallback: str) -> str:
    """A slug guaranteed to be non-empty, falling back when nothing survives."""
    slug = slugify(value)
    return slug if slug else fallback

Install

fune build

With that line in your source, in a Python project (language python in fune.project), fune build resolves it and nothing else, pins them in fune.lock, downloads only the Python package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add text.slugify

Download for Python text.slugify-1.0.0-python.fune · 8,532 bytes sha256 cd99636651945e330070a65713977debca104c5e68509b44cba806d089aa3448

The manifest, vectors and README with only the Python implementation. Install it without the registry with fune add ./text.slugify-1.0.0-python.fune, or fetch it from a terminal with fune pull text.slugify@1.0.0:python.

The whole function, every language, is one file too: text.slugify-1.0.0.fune, 16,855 bytes, sha256 6d01bf54f96ea7650d9b1fb098f56f1fb3119a0164e1d193fb181385a1125b2e. It installs into a project of any language.

Customise it in your app

The seams this capability offers. Put a marker directly above a function of your own and fune build wires it into the built code; the package on the registry is not changed, the built file’s header lists it under CUSTOMISED, and fune hooks lists every hook in the project. How hooks work.

before — your function gets the arguments and returns them, changed or not, or throws to refuse the call.

# fune: before text.slugify

after — your function gets the result and the arguments, and returns the final result.

# fune: after text.slugify

replace — it requires no other capability, so there is no dependency to replace.

step — your function runs at a numbered point inside the function’s body, receives the in-scope values it names as parameters, and may return replacements. List the points with fune show text.slugify --steps.

# fune: step text.slugify after <n|label>

Tests

A version published now needs at least 8 tests for every function, and one that expects the error for each function that throws; the registry refuses it otherwise. fune verify --all runs each case in TypeScript, Python and Rust, and a project runs them again with fune verify. This page lists the cases; it does not run them. The exact JSON is vectors.json.

Case	Arguments		Expected
an ordinary title	Hello World	→	hello-world
something already a slug is left alone	already-a-slug	→	already-a-slug
punctuation, repeated spaces and stray hyphens all collapse to one hyphen	--Héllo, Wörld!--	→	hello-world
accents are dropped, not expanded	Crème Brûlée	→	creme-brulee
sharp s is two letters, so it becomes ss	Straße	→	strasse
ligatures are letters in their own right	Æther & Œuvre	→	aether-oeuvre
thorn and slashed o transliterate	Ølsen Þórsdóttir	→	olsen-thorsdottir
the Windows-1252 carons are covered too	Žižek Šuma	→	zizek-suma
y with diaeresis, upper and lower	Ünicode ÿ Ÿ	→	unicode-y-y
digits survive and brackets do not	Model 3 (2024)	→	model-3-2024

Show the other 11 tests

Case	Arguments		Expected
underscores are separators like any other non-alphanumeric	snake_case_name	→	snake-case-name
a run of hyphens collapses to one	a---b	→	a-b
separators at either end are trimmed	-leading and trailing-	→	leading-and-trailing
mixed case folds down	MIXED Case ÜBER	→	mixed-case-uber
an empty string slugifies to an empty string		→
punctuation alone leaves nothing	!!!???	→
characters outside the table are dropped, keeping the Latin ones	東京 Tokyo	→	tokyo
a title in a script the table does not cover slugifies to empty rather than raising	Ελλάδα	→
emoji are dropped without leaving stray hyphens	🚀 Launch Day 🚀	→	launch-day
a single character is a single character slug	é	→	e
a non-string is a caller bug	42	→	error: slugify needs a string

More from the author

Anything outside that set is dropped, not guessed: Greek, Cyrillic, CJK, emoji and every Latin Extended letter beyond the seven above become separators. A title written entirely in one of those scripts slugifies to the empty string, which is returned rather than raised - the caller knows what its fallback is (an id, a date, a hash) and this function does not.

The map is spelled out character by character rather than left to a Unicode normalisation library, because that is the only way three languages with three different Unicode stacks can be pinned to the same output by one test vector.

Files

Path	Bytes
README.md	1,251
impl/python.py	3,043
impl/rust.rs	4,408
impl/typescript.ts	3,262
vectors.json	2,312