52 lines
2.5 KiB
Markdown
52 lines
2.5 KiB
Markdown
# Word boundary: \b
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A word boundary `pattern:\b` is a test, just like `pattern:^` and `pattern:$`.
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When the regexp engine (program module that implements searching for regexps) comes across `pattern:\b`, it checks that the position in the string is a word boundary.
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There are three different positions that qualify as word boundaries:
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- At string start, if the first string character is a word character `pattern:\w`.
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- Between two characters in the string, where one is a word character `pattern:\w` and the other is not.
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- At string end, if the last string character is a word character `pattern:\w`.
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For instance, regexp `pattern:\bJava\b` will be found in `subject:Hello, Java!`, where `subject:Java` is a standalone word, but not in `subject:Hello, JavaScript!`.
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```js run
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alert( "Hello, Java!".match(/\bJava\b/) ); // Java
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alert( "Hello, JavaScript!".match(/\bJava\b/) ); // null
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```
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In the string `subject:Hello, Java!` following positions correspond to `pattern:\b`:
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So, it matches the pattern `pattern:\bHello\b`, because:
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1. At the beginning of the string matches the first test `pattern:\b`.
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2. Then matches the word `pattern:Hello`.
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3. Then the test `pattern:\b` matches again, as we're between `subject:o` and a comma.
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The pattern `pattern:\bHello\b` would also match. But not `pattern:\bHell\b` (because there's no word boundary after `l`) and not `Java!\b` (because the exclamation sign is not a wordly character `pattern:\w`, so there's no word boundary after it).
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```js run
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alert( "Hello, Java!".match(/\bHello\b/) ); // Hello
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alert( "Hello, Java!".match(/\bJava\b/) ); // Java
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alert( "Hello, Java!".match(/\bHell\b/) ); // null (no match)
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alert( "Hello, Java!".match(/\bJava!\b/) ); // null (no match)
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```
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We can use `pattern:\b` not only with words, but with digits as well.
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For example, the pattern `pattern:\b\d\d\b` looks for standalone 2-digit numbers. In other words, it looks for 2-digit numbers that are surrounded by characters different from `pattern:\w`, such as spaces or punctuation (or text start/end).
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```js run
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alert( "1 23 456 78".match(/\b\d\d\b/g) ); // 23,78
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alert( "12,34,56".match(/\b\d\d\b/g) ); // 12,34,56
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```
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```warn header="Word boundary `pattern:\b` doesn't work for non-latin alphabets"
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The word boundary test `pattern:\b` checks that there should be `pattern:\w` on the one side from the position and "not `pattern:\w`" - on the other side.
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But `pattern:\w` means a latin letter `a-z` (or a digit or an underscore), so the test doesn't work for other characters, e.g. cyrillic letters or hieroglyphs.
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```
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