# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(\b(?'first_word'[a-z](?!([[:alnum:]]*_)(?# the word doesn't contain underscores))(?:[a-z0-9]*)))|(\G(?'trailing_word'[A-Z][a-z0-9]*))/\L${first_word}${trailing_word:+_\L${trailing_word}:}/gm;t' <<< "camelCase
camel
camelCaseCase
camel684Case
ca45melCase
camelCase6523Case
camelCaseCaseCaseCaseCase
camelCCase
PascalCase
// some random commentary with Capitalized letters
camel_Snake_Case
"
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