What is a word boundary in regex?

user_130964 0
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I'm trying to use regexes to match space-separated numbers. I can't find a precise definition of \b ("word boundary"). I had assumed that -12 would be an "integer word" (matched by \b\-?\d+\b) but it appears that this does not work. I'd be grateful to know of ways of .

[I am using Java regexes in Java 1.6]

Example:

Pattern pattern = Pattern.compile("\\s*\\b\\-?\\d+\\s*");
String plus = " 12 ";
System.out.println(""+pattern.matcher(plus).matches());

String minus = " -12 ";
System.out.println(""+pattern.matcher(minus).matches());

pattern = Pattern.compile("\\s*\\-?\\d+\\s*");
System.out.println(""+pattern.matcher(minus).matches());

This returns:

true
false
true
word-boundary regex
13 Answers

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.

enter image description here

user_49906420

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 want to replace all occurrences of the letter 'a' only if this letter ('a') exists at the "Boundary of a word",

In other words: 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 a cat and she's awesome ->

This is e cat end she's ewesome //Result

user_81641160

Reference: Mastering Regular Expressions (Jeffrey E.F. Friedl) - O'Reilly

\b is equivalent to (?<!\w)(?=\w)|(?<=\w)(?!\w)

user_56953000

Word boundary \b is used where one word should be a word character and another one a non-word character. Regular Expression for negative number should be

--?\b\d+\b

check working DEMO

user_51600230

when you use \\b(\\w+)+\\b that means exact match with a word containing only word characters ([a-zA-Z0-9])

in your case for example setting \\b at the begining of regex will accept -12(with space) but again it won't accept -12(without space)

for reference to support my words: https://docs.oracle.com/javase/tutorial/essential/regex/bounds.html

user_72754220
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