GREP_OPTIONS is a global variable that can be set in the local .bashrc or even in env.d that specifies default options for grep. When GREP_OPTIONS="--colour=yes", The output of euse -i mp3 goes like this: loki ~ # euse -i mp3 global use flags (searching: mp3) ************************************************************ no matching entries found local use flags (searching: mp3) ************************************************************ [+ C ] mp3 media-sound/xwax: Sets runtime dependencies to support decoding MP3 audio. egrep: Unmatched [ or [^ grep: Unmatched ) or \) grep: Unmatched [ or [^ [...] Euse should either add: unset GREP_OPTIONS which would solve this for all possible cases or workaround by doing: GREP_OPTIONS+=" --colour=no" Reproducible: Always