Hi @Mohammed ,
No, it is impossible. Column Validation feature is not supported in a multiple line of text column. Only the following field types can be validated with the Column Validation feature: Single Line of Text, Choice, Number, Currency, Date & Time. If a single line of text column is an alternative for you, you can use the formula =IF(ISERROR(FIND("@",[ColumnName])),TRUE)
to restrict user not to type in strings contains "@" in that field.
If you persist on a multiple line of text column, currently you have to inject JS code into SharePoint Online using SPFX. Or you can share your user voice here: Share. Vote. Discuss. It is a place for customers provide feedback about Microsoft SharePoint products. What's more, if a feedback is high voted there by other customers, it will be promising that Microsoft Product Team will take it into consideration when designing the next version in the future.
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