/usr/share/doc/tftpd-hpa/examples/sample.rules is in tftpd-hpa 5.2+20150808-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# Sample rule file for the -m (remapping option)
#
# This file has three fields: operation, regex, remapping
#
# The operation is a combination of the following letters:
#
# r - rewrite the matched string with the remapping pattern
# i - case-insensitive matching
# g - repeat until no match (used with "r")
# e - exit (with success) if we match this pattern, do not process
# subsequent rules
# s - start over from the first rule if we match this pattern
# a - abort (refuse the request) if we match this rule
# G - this rule applies to TFTP GET requests only
# P - this rule applies to TFTP PUT requests only
#
# The regex is a regular expression in the style of egrep(1).
#
# The remapping is a pattern, all characters are verbatim except \
# \0 copies the full string that matched the regex
# \1..\9 copies the 9 first (..) expressions in the regex
# \\ is an escaped \
#
# "#" begins a comment, unless \-escaped
#
ri ^[a-z]: # Remove "drive letters"
rg \\ / # Convert backslashes to slashes
rg \# @ # Convert hash marks to @ signs
rg /../ /..no../ # Convert /../ to /..no../
e ^ok/ # These are always ok
r ^[^/] /tftpboot/\0 # Convert non-absolute files
a \.pvt$ # Reject requests for private files
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