Change quote style mid-string

When building a string in bash, we're not limited to one type of quotes (i.e. single ' or double " quotes) - you can switch mid string. This is especially useful if we require double quotes for part of the string to interpolate a variable, but using double quotes for the entire string would require many other characters to be escaped.

Say we have this grep command for finding yaml code blocks within a markdown file:

grep -E '^[[:space:]]*```ya?ml[[:space:]]*$' example.md

If we want to use a variable for the language part, we can use double quotes for just that part of the string, eliminating the need to escape the backticks:

language='ya?ml'
grep -E '^[[:space:]]*```'"$language"'[[:space:]]*$' test/assets/example.md