I validate the string, but for some reason it does not accept the firebase.
More on the screenshots:
On the validator, everything is fine 
I don’t understand what’s wrong here
In Firebase, the regular expression is passed as a string literal, so all backslashes need to be escaped. Straight slashes must be escaped, as these characters are used to "delimit" a regular expression ("regex delimiters").
Use
.matches(/^steam:\\/\\/joinlobby\\/[0-9]{2,5}\\/[0-9]{10,20}\\/[0-9]{10,20}$/) ^^ ^^ ^^ ^^ ^^ Source: https://ru.stackoverflow.com/questions/711730/
All Articles
\. - Wiktor Stribiżew