How to get all the lines of a file starting from the first one that matches a regex, up to the end

file_from_first_match.sh

tail -n $(grep -n -m 1 "regex to match" "source file" | cut -d : -f1) "source file" > "dest file"

# The above throws away the first line that matches the regex. This will keep that line:
tail -n $(( $(grep -n -m 1 "regex to match" "source file" | cut -d : -f1) - 1)) "source file" > "dest file"

Say we have this file:

line one
line two
yay, line three!
line four
yay, line five!
line six

Run this command:

tail -n $(grep -n -m 1 "yay" "source file" | cut -d : -f1) "source file"

to get this output:

line four
yay, line five!
line six

Run the second version to get this output:

yay, line three!
line four
yay, line five!
line six