When Do Word Boundaries Occur in PHP Regular Expressions?

Word Boundary Semantics in PHP Regular Expressions

In PHP, word boundaries are implemented using the b metacharacter, which matches transitions between word characters (w) and non-word characters (W). However, its behavior can be nuanced, as exemplified by your provided test cases.

Unexpected Word Boundaries

In your test cases, you expected the following results:

preg_match("/(^|\b)@nimal/i", "something@nimal", $match); // false
preg_match("/(^|\b)@nimal/i", "something!@nimal", $match); // true

But the actual results were reversed:

preg_match("/(^|\b)@nimal/i", "something@nimal", $match); // true
preg_match("/(^|\b)@nimal/i", "something!@nimal", $match); // false

This anomaly occurs because the b metacharacter matches at the transition from a word character (w) to a non-word character (W). In the first case, "something@nimal," the word boundary occurs between "g" (a word character) and "@" (a non-word character), leading to a match. However, in the second case, "something!@nimal," there is no word boundary between "!" and "@" because both are non-word characters.

Matching Word Beginnings

To match words that start with a specific sequence, you need to ensure that there is a word boundary (b) before your target sequence. For example, to match words starting with "@nimal," you would use the following regex:

preg_match("/(\b)@nimal/i", "something@nimal", $match); // true
preg_match("/(\b)@nimal/i", "something!@nimal", $match); // false

In this regex, the word boundary (b) will ensure that there is a transition from a word character to a non-word character before "@nimal." This will match words starting with "@nimal," but not words that have "@nimal" in the middle, such as "ducat."

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