Placeholders are rules that can be added to an account to protect text that should not be translated and that the users wants to protect from the translation team.
Placeholders can be added from the Settings menu:
Placeholders apply only to certain file formats - INC, JSON, MQXLIFF, PHP, PO, RESX, SDLXLIFF, STRINGS, TJSON, TTX, XLF, XLIFF, XLSX, XML.
Placeholders are created using what is called regular expressions. These are sequences of characters that are used to define a search pattern. More information can be found here. Once the result of the search is found, the matching string is converted to a Smartcat internal tag that cannot be edited. For non-programmers, regular expression can be difficult to write and the support team can help users if needed.
The following website can be used to learn and test regular expressions - https://regexr.com/