grep1-3  Exercise 3: Checking a Basic-to-Extended Translation
=============================================================

cat > lines.txt <<'EOF'
cat
concatenate
The cat sat on the mat
cot cut cat c.t
color colour
a+b=c
aaa
abab abab
price: $19.99
call 555-0142 now
2026-10-08 ERROR disk full
2026-10-08 WARN low memory
the the end
[note] keep this
path C:\temp\file
hello world
   indented line

file.txt and fileXtxt
end.
EOF

check() {
  if diff <(grep -o "$1" lines.txt) <(grep -oE "$2" lines.txt) > /dev/null; then
    echo "same:      basic '$1'   extended '$2'"
  else
    echo "DIFFERENT: basic '$1'   extended '$2'"
  fi
}

# four correct translations
check 'colou\?r'                      'colou?r'
check 'ERROR\|WARN'                   'ERROR|WARN'
check '\(ab\)\{2\}'                   '(ab){2}'
check '[0-9]\{3\}-[0-9]\{4\}'         '[0-9]{3}-[0-9]{4}'
# two that look right and are not
check 'a+b'                           'a+b'
check 'file.txt'                      'file\.txt'

Output:

same:      basic 'colou\?r'   extended 'colou?r'
same:      basic 'ERROR\|WARN'   extended 'ERROR|WARN'
same:      basic '\(ab\)\{2\}'   extended '(ab){2}'
same:      basic '[0-9]\{3\}-[0-9]\{4\}'   extended '[0-9]{3}-[0-9]{4}'
DIFFERENT: basic 'a+b'   extended 'a+b'
DIFFERENT: basic 'file.txt'   extended 'file\.txt'

WHY THIS WORKS AS AN ANSWER
---------------------------
The function runs the basic pattern without -E and the extended one with it, using -o so that the comparison is of what matched, not just which lines. diff <(...) <(...) compares the two outputs as if they were files (bash, not plain sh); the exit status of diff drives the if.
The first four are genuine translations: every backslash operator in the basic form loses its backslash. The fifth is the trap from the chapter: the same text a+b is a literal plus in basic and 'one or more a' in extended, so the outputs differ (the basic one finds 'a+b', the extended one finds the 'ab' in 'abab').
The sixth is a different slip: 'file.txt' written the same way in both would be the same, but translating it as 'file\.txt' changes it, because the unescaped dot also matched 'fileXtxt'. A translation that is correct for the engine but changes your intent is a difference worth knowing about.