Difference between \b and \B in regex

user_346293 0
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I am reading a book on regular expression and I came across this example for \b:

The cat scattered his food all over the room.

Using regex - \bcat\b will match the word cat but not the cat in scattered.

For \B the author uses the following example:

Please enter the nine-digit id as it

appears on your color - coded pass-key.

Using regex \B-\B matches - between the word color - coded. Using \b-\b on the other hand matches the - in nine-digit and pass-key.

How come in the first example we use \b to separate cat and in the second use \B to separate -? Using \b in the second example does the opposite of what it did earlier.

Please explain the difference to me.

EDIT: Also, can anyone please explain with a new example?

regex
9 Answers

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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.

user_68252880

\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']
user_68194420
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