# Just prints one big long string, and pipes it into sed.
echo "slartibartfast q;  int aslartibartfast;  slartibartfast b, c;  int d;" |
        # sed 's/a/b/g' searches for all a, replaces with b.
        # a can be an entire expression, not just a literal string.
        # Just 's/slartibartfast/qwerty/g' would change our
        # 'aslartibartfast' to 'aqwerty', which is wrong, so we need to check
        # where it is too.
        #
        # [ ;] matches either a semicolon or a space, so
        # [ ;]slartibartfast[ ;] would catch it in the middle of a line, but
        # not the beginning or end.
        #
        # ^ means beginning, $ means end, | means 'or', () group things.  So
        " ([ ;]|^) would mean the beginning of the line or space or semicolon.
        # ([ ;]|^)slartibartfast([ ;]|$) should catch it anywhere in the line.
        #
        # Except if we search-replace that, we're stripping out lots of
        # whitespace and semicolons -- you need those.  You can put back
        # anything you find in () with \1, \2, ... for match 1, match 2 ...
        #
        # So:
        #
        # s/([ ;]|^)slartibartfast([ ;]|)/\1qwerty\2/g'
        # ...tells it to replace ;slartibartfast; with qwerty and put the ;'s back.
        #
        # Except, sed is horrible and won't understand ( | ) unless
        # you put \ in front of them, which makes otherwise-simple expressions
        # capable of blinding small dogs and the elderly.
        #
        #If you have sed -r, you can leave them as-is.
        sed 's/\([ ;]\|^\)slartibartfast\([ ;]\|$\)/\1qwerty\2/g'
# And this is what it prints.  See how it left aslartibartfast alone.
qwerty q;  int aslartibartfast;  qwerty b, c;  int d;
$