Do not check that Source is a valid regex in case of Target (explicit)
transformation. Source may contain special symbols that may cause an
incorrect `invalid regex` error.
Note that source and exactly one of [Target, Transform] must be
provided.
`Target (explicit transformation)`: In this kind of rule `Source` is
treated as a symbol name and is matched in its entirety. `Target` field
will denote the symbol name to transform to.
`Transform (pattern transformation)`: This rule treats `Source` as a
regex that should match the complete symbol name. `Transform` is a regex
specifying the name to transform to.
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