# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(AVE)\s+(?=\d+\b)/$1 \/ /gm;t' <<< "10 AVE 2 ST
NORTH 100 AVE 5 TERR
S 39 AVE NW
S 39 AVE 1R
S 39 AVE 234 S"
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html