Why Do Regular Expression Word Boundaries (\b) Misbehave with Special Characters?

Word Boundary Behavior for Expressions with Special Characters

When using regular expressions to match words, the b boundary pattern can sometimes produce unexpected results, particularly when the word contains special characters.

Consider the phrase "test Sortesindex[persons]{Sortes} text." Matching the full phrase using bb should prevent matches on phrases like "Sortes" alone. However, this does not occur due to certain characteristics of word boundaries.

b Behavior with Special Characters

Formally, b matches the boundary between a word character (w) and a non-word character (W), or the beginning/end of the string. When the word boundary pattern is immediately followed by a special character like [ or }, the b only requires a word character after the special character.

Solution: Adaptive or Unambiguous Word Boundaries

To address this issue, adaptive word boundaries or unambiguous word boundaries can be used.

Adaptive word boundaries handle any combination of word and non-word characters on either side of the pattern:

re.search(r'(?:(?!\w)|\b(?=\w)){}(?:(?<p><strong>Unambiguous word boundaries</strong> require no word characters on either side of the pattern:</p><pre class="brush:php;toolbar:false">re.search(r'(?<h3 id="Choosing-the-Right-Boundary-Type">Choosing the Right Boundary Type</h3><p>Adaptive word boundaries are more versatile but allow leading and trailing non-word characters. Unambiguous word boundaries are stricter, ensuring no surrounding word characters.</p><h3 id="Customization">Customization</h3><p>The lookaround patterns used for adaptive and unambiguous word boundaries can be customized based on the specific requirements of the matching task. For instance, to only match non-letters around the pattern, use [^Wd_] instead of w.</p>

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