Roblox has a chat filter that prohibits players from seeing or sending profanity and restricts personally identifiable information, which replaces the now-discontinued Safe Chat that was previously used to prevent young players from sending messages that were not pre-programmed. Currently, the censored text is replaced with a hashtag symbol (#).
Historically, Roblox used a hidden word blacklist that came bundled with the Roblox Player, stored in the file diogenes.fnt. Speaking any word contained in that list would usually render an instant ban from the website. The player would then be disconnected from whatever game they're playing. Repeated infractions using the chatbox would usually render a termination. Only 13+ accounts could chat or post on the website (forums, comments, etc.), as well as being able to chat freely in-game. Guests were restricted to the Safe Chat, as well as <13 (under 13) users (Unless disabled in the account's respective Parent account).
Today, the text filter is provided by the CommunitySift service. Since CommunitySift uses multiple rules and text classification techniques to filter phrases, the lists in this article are not representative of the entire range of words that are filtered. The system is a controlled neural network, which is constantly studying what users say so it can learn and evolve with the language as new bypasses and idioms are created. Errors happen frequently, but the occurrence of such has diminished over time.
Alongside filtering potential profanity from the chats, the filter also attempts to censor private information such as ages, telephone numbers and surnames, so that the website can continue to be COPPA-compliant by protecting users from unknowingly sharing their personally identifiable information (or PII).
Criticism
False positives
The filtering system, at times, may detect false positives in sentences that could be considered otherwise acceptable. Words used in common sentences such as "the", "my", "town" sometimes get picked up by the filter and get censored. The number of hits has reduced in the last years, as the filter evolved to detect what is a normal conversation and what is not, based on feedback given by Roblox Staff and by common users, although false positives still happen frequently. Those are most noticeable when searching through the catalog or when looking for a username.
Blacklisted Numbers
In February 2017, <13 accounts had all forms of numbers blacklisted from being seen and sent permanently for security reasons - namely, to avoid underage users sending messages with personally identifiable information such as age, addresses and telephone numbers. This has caused quite a commotion in the Roblox community, and criticism flew around from all parts.
During some time, LGBT culture words such as "gay", "homosexual", "lesbian", "bisexual", "transsexual" and the alike were temporarily blacklisted by the filter automatically, severely upsetting many users who identified as them. This has since been fixed, and now the filter only blocks these words when used in negative connotations.
December 3 incident
On December 3, 2019, the chat filer completely glitched, censoring all sorts of words for both <13 and 13+ users, which generated a lot of controversy amongst the Roblox fanbase. This was fixed after about half an hour after it started.