no such variable "regexp"

On (info "(mailutils) Mail Variables")

'regex'

     Type: Boolean.
     Default: True.

     Setting this to 'True' enables use of regular expressions in
     '/.../' message specifications.

But on (info "(mailutils) Specifying Messages")
Header match
     The "header match" is a string in the form:

          [HEADER:]/STRING/

     It selects all messages that contain header field HEADER matching
     given REGEXP.  If the variable 'regexp' is set, the STRING is
                                     ^^^^^^

regex or regexp... apparently "regex" is the real variable, and regexp
is a typo.