...
To create a new username as an output, use the variables and other symbols to build the replacement. In the given example, having the replacement set to $1$4$5 will transform from john.doe_teacher@example.com to john.doe@example.com.
Change case of username
When using just-in-time provisioning and regular expressions for username lookup, you may want to control what case usernames get. By using \L (lowercase) and \U (uppercase) in front of $1 and other replacements (see above examples), you may instruct what casing the username for the newly created user will be.
See more details on the language of regular expressions here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
...