The special characters \b and \B go hand in hand in regular expressions:
\b matches at word boundaries; and\B matches inside words.For these two characters, the default “word characters” are alphanumeric characters and the underscore.
By combining \b and \B at the beginning or end of a pattern, you get to match standalone words, prefixes, suffixes, and infixes!
The table below shows some examples of sentences that all contain the substring "legal" along the rows.
The columns show whether different patterns that use the special characters \b and \B would match against those sentences.
r"legal" |
r"\blegal\b" |
r"\blegal\B" |
r"\Blegal\b" |
r"\Blegal\B" |
|
|---|---|---|---|---|---|
"Criticism is legal." |
✅ | ✅ | ❌ | ❌ | ❌ |
"He's legally blind." |
✅ | ❌ | ✅ | ❌ | ❌ |
"Theft is illegal." |
✅ | ❌ | ❌ | ✅ | ❌ |
"He obtained that illegally." |
✅ | ❌ | ❌ | ❌ | ✅ |
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