...
Available for the "Replace using a regular expression" operation. This text will used to replace regular expression matches in the variable's value. This may contain back references to captured groups in the regular expression pattern, see Substitutions in Regular Expressions. Note these need to be escaped e.g. $1 => $$1 if expanding.
Expand
...
expressions in replacement value
If this option is ticked, then the Replace With will be expanded (if there's any expressions in it) before it is used to replace regular expression matches in the variable's value.
...