A username

Addresses and identifiers

A username is a short label chosen by whoever registered it, and the only thing in the world it identifies is an account.

The shape

Usernames are the loosest shape in this family. There is no public rule to appeal to, only whatever the registration form happened to accept, and forms differ. What can be said is that they are short, they are chosen rather than derived, and they are typed by people, which is where every problem on this card starts.

Three properties do most of the damage. Case may or may not be significant, and the same string in capitals may name the same account or a different one depending on the system. Punctuation is usually restricted to a small set such as an underscore, a hyphen, a full stop, and the restriction varies. Whitespace at either end is invisible on screen but not always discarded, so a label that was copied with a trailing space is a different string from the label without it, even though the two look the same in every font.

Length

short, bounded by whatever the form allowed rather than by a standard

Alphabet

letters and digits, plus a small set of punctuation that varies by system

Case

sometimes significant, sometimes folded; the label itself does not say which

Interior spaces

usually refused, occasionally allowed, never safe to assume

Edges

no leading or trailing whitespace, though copies pick it up constantly

Width on screen

not a reliable guide to the number of characters

examplexxxx
example_xxxx
example.xxxx
three placeholders. all short, all typed, none derived from anything

The characters you cannot see

Some characters occupy no width when drawn. A zero width space, a zero width joiner, a mark that only affects text direction: all of them are real characters that a text field will store and a screen will not show. Slip one into the middle of a label and you get a second string that renders identically to the first at every size, in every font, on every device.

This is the one place where the eye is genuinely helpless, and admitting that is more useful than a procedure that cannot work. What does work is anything that reports position or count rather than appearance. Press the arrow key along the label and watch the caret: it will pause on a character that shows nothing. Select the label and look at where the highlight starts and stops, since a highlight has width even where a glyph does not. Or compare against a copy you already had, in a field that shows you the length.

The same trick with fewer moving parts is a name that differs by an ordinary character in a position nobody reads: a doubled letter in a long run, a hyphen where an underscore used to be, a trailing digit. Two accounts really can exist one character apart, and the shorter the label the harder that is to notice.

How to check it by eye

What a wrong shape means

Nearly every broken username is a copying artefact. A double click selects a word and stops at punctuation, so half a label comes across. A tap and drag on a phone takes a space along with it. A label at the end of a sentence arrives with the full stop attached. A quoted label arrives with one quotation mark. None of these are interesting, and all of them produce a string that will not match anything.

A label that differs from your copy by one visible character is the case worth slowing down for, and even then the honest reading is that you have two strings rather than that you have caught something. Names are reassigned, people register several, and a system that folds case will treat two of your three copies as the same label anyway. What the difference does mean is that you cannot treat the two as interchangeable, and that a message addressed to one of them is not addressed to the other.

example_xxxx
twelve characters, nothing before or after

 example_xxxx.
a leading space and a full stop from the sentence

The two render almost identically in a message and are different strings everywhere it matters. The marked characters were never part of the label; they came from the text around it.

What a correct shape does not prove

This is the section the card exists for. A username can be clean, consistent, free of hidden characters and identical to the copy you were given, and still support none of the following.

• Who is behind it. A label names an account. An account is a row in a database, and rows are opened, shared, sold, abandoned and taken over without the label changing by a single character.
• That it is the same account as last time. Names get released and reused, and a label you recognise may sit above a record that has nothing to do with the one you remember.
• That it is the same person as on some other page. Labels do not span systems. Two identical names in two places are two names that happen to match, and matching is not evidence of anything.
• That anyone answering to it is the person who registered it. Access and ownership are separate questions, and the label does not touch either.
• That there is not a second account one character away. The shape of a label says nothing about what other labels exist near it.
• Anything at all about conduct or standing. There is no history inside a string, and this site publishes no assessment of any account.

What this card is not

Deliberate imitation using characters that resemble other characters is treated on the lookalike host card, which sets out which substitutions are possible and which are folklore. A label with an at sign in it is a different shape entirely and belongs to the email shaped string card. Anything about credentials attached to an account, rather than the label on it, starts at the session hub. This card also makes no claim about any account on any market, and there is no register of names here to look anything up in.