Since Hudson 1.349, console notes can be embedded into the console output file. These notes are stored in an encoded format and are intended to be stripped out before the console is displayed.

The build log regex functionality, since it reads from the console output file directly, puts these encoded notes into the email.

http://hudson.361315.n4.nabble.com/Timestamper-plugin-and-email-ext-build-log-regex-produces-garbage-in-email-td2530211.html#a2530211

These console notes can be stripped out by calling ConsoleNote.removeNotes(String) (introduced in Hudson 1.350) on each line as the BuildLogRegexContent class reads it from the log.