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03-more/08-regular-expressions-javascript/15-regexp-word-boundary/article.md

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+# Word boundary
+
+Another position check is a *word boundary* <code class="pattern">\b</code>. It doesn't match a character, but matches in situations when a wordly character follows a non-wordly or vice versa. A "non-wordly" may also be text start or end.
+[cut]
+For example, <code class=pattern">\bdog\b</code> matches a standalone <code class="subject">dog</code>, not <code class="subject">doggy</code> or <code class="subject">catdog</code>:
+
+```js
+//+ run
+showMatch( "doggy catdog dog", /\bdog\b/ ) // "dog"
+```
+
+Here, <code class="match">dog</code> matches, because the previous char is a space (non-wordly), and the next position is text end.
+
+Normally, <code class="pattern">\w{4}</code> matches 4 consequent word characters.
+If the word is long enough, it may match multiple times:
+
+```js
+//+ run
+showMatch( "Boombaroom", /\w{4}/g) // 'Boom', 'baro'
+```
+
+Appending <code class="pattern">\b</code> causes <code class="pattern">\w{4}\b</code> to match only at word end:
+
+```js
+//+ run
+showMatch( "Because life is awesome", /\w{4}\b/g) // 'ause', 'life', 'some'
+```
+
+**The word boundary <code class="pattern">\b</code> like <code class="pattern">^</code> and <code class="pattern">$</code> doesn't match a char. It only performs the check.**
+
+Let's add the check from another side, <code class="pattern">\b\w{4}\b</code>:
+
+```js
+//+ run
+showMatch( "Because life is awesome", /\b\w{4}\b/g) //  'life'
+```
+
+Now there is only one result <code class="match">life</code>.
+
+<ol>
+<li>The regexp engine matches first word boundary <code class="pattern">\b</code> at zero position:
+<img src="boundary1.png">
+</li>
+<li>Then it successfully matches <code class="pattern">\w{4}</code>, but fails to match finishing <code class="pattern">\b</code>.
+
+<img src="boundary2.png">
+
+So, the match at position zero fails. 
+</li>
+<li>The search continues from position 1, and the closest <code class="pattern">\b</code> is right after <code class="subject">Because</code> (position 9):
+
+<img src="boundary3.png">
+
+Now <code class="pattern">\w{4}</code> doesn't match, because the next character is a space.
+</li>
+<li>The search continues, and the closest <code class="pattern">\b</code> is right before <code class="subject">life</code> at position 11.
+
+<img src="boundary4.png">
+
+Finally, <code class="pattern">\w{4}</code> matches and the position check <code class="pattern">\b</code> after it is positive. We've got the result.
+</li>
+<li>The search continues after the match, but doesn't yield new results.</li>
+</ol>
+
+**The word boundary check <code class="pattern">/\b/</code> works only for words in latin alphabet,** because it is based on <code class="pattern">\w</code> as "wordly" chars. Sometimes that's acceptable, but limits the application range of the feature. 
+
+And, for completeness..
+**There is also an inverse check <code class="pattern">\B</code>, meaning a position other than <code class="pattern">\b</code>.** It is extremely rarely used.
+