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Regular expressions are used for text pattern-matching, whereas plain text expressions can be used both to do literal character matching and provide output values. In simple terms, writing a plain text expression for a string valued field on a stream would result in a (that) string being output to the database, but a regular expression would have no meaning in this context.
Examples:
If you want the string abc"def.
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Note how this contrasts to the regular expression case, where a double-backslash is used to escape special characters.