Where is lookaround regex supported and where isn't it?

Question

Trying to improve my regex skills, I wanted to learn about lookahead and lookbehind expressions. On my Archlinux system I tried the following:

a=ab;if [[ $a =~ [a-z](?=b) ]]; then echo "Y";else echo "N";fi

Which, as far as I understand it, should match and thus echo out a "Y", but doesn't.

echo ab |sed 's/[a-z](?=b)/x/'

...also doesn't seem to match. grep doesn't seem to lookaround either, but pcregrep does. I also tried several attempts on quoteing and/or escaping the expressions, to no avail.

I'm a little confused, now. Could someone please clarify where lookaround, which doesn't seem that exotic judging from the number of mentions in tutorials, can actually be used? Or did I just mess up escaping my expressions?

Answer 1

Lookaround assertions aren't supported by basic or extended posix regular expressions which are available in bash or sed.

A good tool to test is GNU grep which supports the -P option for perl compatible regular expressions. Like this:

grep --color=auto -P '[a-z](?=b)' <<< 'ab'

Even a greater resource are online regex testing tools like https://regex101.com/

Answer 2

You should distinguish between basic and extended Regular Expressions.

In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; They need to be escaped to get their "regex" meaning.

On the other hand, in the extended Regular Expressions, these characters get their "regex" meaning.

If you grep --help, you'll get:

-E, --extended-regexp PATTERN is an extended regular expression (ERE)

Note that grep doesn't support look-arounds, it's supported in pcregrep.