Well, sed supports the y command (character substitution), but since there is not a 1-to-1 mapping, I don't think you can apply that here.
http://docs.splunk.com/Documentation/Splunk/latest/SearchReference/Rex
http://docs.splunk.com/Documentation/Splunk/5.0.2/Data/Anonymizedatausingconfigurationfiles#Through_a_sed_script
However, you can put all of your regexes into props.conf SEDCMD statements, or through transforms, which would make the alteration permanent (may not be compatible with any requirements of keeping data in its original format, if you have such).
http://docs.splunk.com/Documentation/Splunk/latest/Admin/Propsconf
http://docs.splunk.com/Documentation/Splunk/5.0.2/Data/Anonymizedatausingconfigurationfiles#Through_a_regex_transform
Hope this helps,
Kristian
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