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When a user logs in via Kerberos it may be that the format of the username does not match your existing username format in your Atlassian product. Then you may transform the username either by using regular expressions, by username mappings file or by adding an additional domain name to the username lookup (selectable in the upper drop-down in the image above).
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The default behavior is that the transformation will be tried first and then additional lookups will be done with combinations of the username in uppercase and lowercase, and also with and without domain. Turn this switch on to only lookup based on the transformation rule.
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Lookup usernames from mapping file
As screenshot below you may introduce a mapping file on JSON format to map if your needs for changing the usernames. This can be convenient when your username transformation needs do not follow a pattern that could be expressed by a regular expression.
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