What are the differences between regular expression syntaxes for different tools? [closed]

Question

Different tools implement regular expressions differently. For example to match "foo" or "bar":

printf "%s\n" foo bar baz food | grep -o '\<\(fo\+\|bar\)\>'
printf "%s\n" foo bar baz food | awk '/\<(fo+|bar)\>/'
printf "%s\n" foo bar baz food | sed -n '/\<\(fo\+\|bar\)\>/p'
printf "%s\n" foo bar baz food | sed -nr '/\<(fo+|bar)\>/p'

Where are these differences documented?

Answer 1

Score! I'm so happy to have found this page:
https://www.gnu.org/software/gnulib/manual/html_node/Regular-expression-syntaxes.html

14.8 Regular expression syntaxes

Gnulib supports many different types of regular expressions; although the underlying features are the same or identical, the syntax used varies. The descriptions given here for the different types are generated automatically.

• awk regular expression syntax
• egrep regular expression syntax
• ed regular expression syntax
• emacs regular expression syntax
• gnu-awk regular expression syntax
• grep regular expression syntax
• posix-awk regular expression syntax
• posix-basic regular expression syntax
• posix-egrep regular expression syntax
• posix-extended regular expression syntax
• posix-minimal-basic regular expression syntax
• sed regular expression syntax