Python regex: Explain why expression not matching

I am using regex library to find words that are in between specific other words, for example, I want to match "world" if and only if a greeting precedes it and punctuation follows. To avoid matching word prefixes and suffixes, I added the additional condition [^a-zA-Z]. However, once I add these, regex cannot match the word anymore:

>>> import regex

>>> pat = regex.compile("(?<=[^a-zA-Z](hello|hi)\s+)world(?=\s*[!?.][^a-zA-Z])")

>>> list(pat.finditer("hello world!"))
[]

>>> pat = regex.compile("(?<=\b(hello|hi)\s+)world(?=\s*[!?.]\b)")

>>> list(pat.finditer("hello world!"))
[]

>>> pat = regex.compile("(?<=(hello|hi)\s+)world(?=\s*[!?.])")

>>> list(pat.finditer("hello world!"))
[<regex.Match object; span=(6, 11), match='world'>]

How can this be explained? How to make sure to match whole words in the look ahead and behind sections?

Solution

The reason is that when using (?<= and (?= there has to be present on the left and right what you specify.

Note that there is no word boundary after [!?.]\b when there is not a word character following any of the punctuation chars.

You could write the pattern as:

(?<=\b(?:hello|hi)\s+)world(?=\s*[!?.](?!\S))

Explanation

(?<= Positive lookbehind, assert that to the left is
- \b(?:hello|hi)\s+ Match either the word hello or hi and 1+ whitespace chars
) Close lookbhehind
world Match literally
(?= Positive lookahead, assert that to the right is
- \s*[!?.] Match optional whitespace chars and one of ! ? .
- (?!\S) Assert a whitespace boundary to the right
) Close the lookahead

Or asserting a whitespace boundary to the left instead of the word boundary:

(?<=(?<!\S)(?:hello|hi)\s+)world(?=\s*[!?.](?!\S))

Regex demo