# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/# Keys are pretty much "any text before the '=' symbol"
(?P<key>[^=]+)
# Match the '=' symbol
=
(?P<value>
# Lazy match
.*?
# Look-ahead (don't move the "regex cursor", just "peek forward")
(?=
# Match a '_' that has no other '_' between it and the '=' character, or...
_(?=[^_=\n]+=)
|
# ... or the end of the line
$
)
)
# Actually "consume" the '_' character if it exists (or just match the end-of-line)
(?:_|$)/\1 = \2\n/gmx;t' <<< "str=string_number=12345_device=phone_color=blue
color=aqua_marine_name=matt_number=123_456_879_lastname=blahblah"
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