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)){}(?:(?<=\w)\b|(?<!\w))'.format(re.escape('Sortes\index[persons]{Sortes}')), 'test Sortes\index[persons]{Sortes} test')

Unambiguous word boundaries require no word characters on either side of the pattern:

re.search(r'(?<!\w){}(?!\w)'.format(re.escape('Sortes\index[persons]{Sortes}')), 'test Sortes\index[persons]{Sortes} test')

Choosing the Right Boundary Type

Adaptive word boundaries are more versatile but allow leading and trailing non-word characters. Unambiguous word boundaries are stricter, ensuring no surrounding word characters.

Customization

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.

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