"The process name used for matching is limited to the 15 characters present in the output of /proc/pid/stat. Use the -f option to match against the complete command line, /proc/pid/cmdline."
Therefore, this is not a bug, even though it is arguable that the behaviour of pgrep(1)'s default invocation is not intuitive. Accordingly, this can be closed as invalid.
<https:/ /bugs.launchpad .net/ubuntu/ +source/ procps/ +bug/295876/ comments/ 3> led me to review pgrep(1)'s man page (which it references).
Under 'NOTES', the man page states:
"The process name used for matching is limited to the 15 characters present in the output of /proc/pid/stat. Use the -f option to match against the complete command line, /proc/pid/cmdline."
Therefore, this is not a bug, even though it is arguable that the behaviour of pgrep(1)'s default invocation is not intuitive. Accordingly, this can be closed as invalid.