What is a word boundary in regex?

Question

A word boundary, in most regex dialects, is a position between \w and \W (non-word char), or at the beginning or end of a string if it begins or ends (respectively) with a word character ([0-9A-Za-z_]).

So, in the string "-12", it would match before the 1 or after the 2. The dash is not a word character.

In the course of learning regular expression, I was really stuck in the metacharacter which is \b. I indeed didn't comprehend its meaning while I was asking myself "what it is, what it is" repetitively. After some attempts by using the website, I watch out the pink vertical dashes at the every beginning of words and at the end of words. I got it its meaning well at that time. It's now exactly word(\w)-boundary.

My view is merely to immensely understanding-oriented. Logic behind of it should be examined from another answers.

A word boundary can occur in one of three positions:

1. Before the first character in the string, if the first character is a word character.
2. After the last character in the string, if the last character is a word character.
3. Between two characters in the string, where one is a word character and the other is not a word character.

Word characters are alpha-numeric; a minus sign is not. Taken from Regex Tutorial.

A word boundary is a position that is either preceded by a word character and not followed by one, or followed by a word character and not preceded by one.

I would like to explain Alan Moore's answer

A word boundary is a position that is either preceded by a word character and not followed by one or followed by a word character and not preceded by one.

Suppose I have a string "This is a cat, and she's awesome", and I am supposed to replace all occurrence(s) the letter 'a' only if this letter exists at the "Boundary of a word", i.e. the letter a inside 'cat' should not be replaced.

So I'll perform regex (in Python) as

re.sub(r"\ba","e", myString.strip()) //replace a with e

so the output will be

This is e cat end she's ewesome

I talk about what \b-style regex boundaries actually are here.

The short story is that they’re conditional. Their behavior depends on what they’re next to.

# same as using a \b before:
(?(?=\w) (?<!\w)  | (?<!\W) )

# same as using a \b after:
(?(?<=\w) (?!\w)  | (?!\W)  )

Sometimes that isn’t what you want. See my other answer for elaboration.