Difference between \b and \B in regex

Question

The confusion stems from your thinking \b matches spaces (probably because "b" suggests "blank").

\b matches the empty string at the beginning or end of a word. \B matches the empty string not at the beginning or end of a word. The key here is that "-" is not a part of a word. So <left>-<right> matches \b-\b because there are word boundaries on either side of the -. On the other hand for <left> - <right> (note the spaces), there are not word boundaries on either side of the dash. The word boundaries are one space further left and right.

On the other hand, when searching for \bcat\b word boundaries behave more intuitively, and it matches " cat " as expected.

\b is a zero-width word boundary. Specifically:

Matches at the position between a word character (anything matched by \w) and a non-word character (anything matched by [^\w] or \W) as well as at the start and/or end of the string if the first and/or last characters in the string are word characters.

Example: .\b matches c in abc

\B is a zero-width non-word boundary. Specifically:

Matches at the position between two word characters (i.e the position between \w\w) as well as at the position between two non-word characters (i.e. \W\W).

Example: \B.\B matches b in abc

See regular-expressions.info for more great regex info

With a different example:

Consider this is the string and pattern to be searched for is 'cat':

text = "catmania thiscat thiscatmaina";

Now definitions,

'\b' finds/matches the pattern at the beginning or end of each word.

'\B' does not find/match the pattern at the beginning or end of each word.

Different Cases:

Case 1: At the beginning of each word

result = text.replace(/\bcat/g, "ct");

Now, result is "ctmania thiscat thiscatmaina"

Case 2: At the end of each word

result = text.replace(/cat\b/g, "ct");

Now, result is "catmania thisct thiscatmaina"

Case 3: Not in the beginning

result = text.replace(/\Bcat/g, "ct");

Now, result is "catmania thisct thisctmaina"

Case 4: Not in the end

result = text.replace(/cat\B/g, "ct");

Now, result is "ctmania thiscat thisctmaina"

Case 5: Neither beginning nor end

result = text.replace(/\Bcat\B/g, "ct");

Now, result is "catmania thiscat thisctmaina"

Hope this helps :)

The metacharacter \b is an anchor like the caret and the dollar sign. It matches at a position that is called a "word boundary". This match is zero-length.

There are three different positions that qualify as word boundaries:

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

\B is the negated version of \b. \B matches at every position where \b does not. Effectively, \B matches at any position between two word characters as well as at any position between two non-word characters.

Source: http://www.regular-expressions.info/wordboundaries.html

\b matches a word-boundary. \B matches non-word-boundaries, and is equivalent to [^\b](?!\b) _{^{(thanks to @Alan Moore for the correction!)}}. Both are zero-width.

See http://www.regular-expressions.info/wordboundaries.html for details. The site is extremely useful for many basic regex questions.

Source © Copyright RexEgg.com

Word Boundary: \b*

The word boundary \b matches positions where one side is a word character (usually a letter, digit or underscore—but see below for variations across engines) and the other side is not a word character (for instance, it may be the beginning of the string or a space character).

The regex \bcat\b would, therefore, match cat in a black cat, but it wouldn't match it in catatonic, tomcat or certificate. Removing one of the boundaries, \bcat would match cat in catfish, and cat\b would match cat in tomcat, but not vice-versa. Both, of course, would match cat on its own.

Not-a-word-boundary: \B

\B matches all positions where \b doesn't match. Therefore, it matches:

✽ When neither side is a word character, for instance at any position in the string $=(@-%++) (including the beginning and end of the string)

✽ When both sides are a word character, for instance between the H and the i in Hi!

This may not seem very useful, but sometimes \B is just what you want. For instance,

✽ \Bcat\B will find cat fully surrounded by word characters, as in certificate, but neither on its own nor at the beginning or end of words.

✽ cat\B will find cat both in certificate and catfish, but neither in tomcat nor on its own.

✽ \Bcat will find cat both in certificate and tomcat, but neither in catfish nor on its own.

✽ \Bcat|cat\B will find cat in embedded situation, e.g. in certificate, catfish or tomcat, but not on its own.

\b is used as word boundary

word = "categorical cat"

Find all "cat" in the above word

without \b

re.findall(r'cat',word)
['cat', 'cat']

with \b

re.findall(r'\bcat\b',word)
['cat']