I only know a way to do this by creating two calculated fields. One for the a to b and the next for the c to d. The one for c to d would use the a to b field as the source data field.
The regex is REGEXP_REPLACE([State],'a','b')
and then REGEXP_REPLACE([a to b],'c','d') to get the final output.
If you want to include the case of the letters to include Caps, then you can do this: REGEXP_REPLACE([a to b],'(?i)c','d') - The modifier (?i) forces a case insensitive search.
I hope that helps
This example workbook is done in Tableau 10.5.3.
regex replace sample.twbx 337.7 KB