Why special characters like = or " break PHP regexp when using \b word boundary?

Question

this is a follow up after reading How to specify "Space or end of string" and "space or start of string"?

From there, it states means to match a word

Answer 1

Background

From the regular-expressions.info Word boundaries page:

The metacharacter \b is an anchor like the caret and the dollar sign. It matches at a position that is called a "word boundary". This match is zero-length.

There are three different positions that qualify as word boundaries:
- Before the first character in the string, if the first character is a word character.
- After the last character in the string, if the last character is a word character.
- Between two characters in the string, where one is a word character and the other is not a word character.

A very good explanation from nhahtdh post:

A word boundary \b is equivalent to:

(?:(?

Which means:

• Right ahead, there is (at least) a character that is a word character, and right behind, we cannot find a word character (either the character is not a word character, or it is the start of the string).

  OR

• Right behind, there is (at least) a character that is a word character, and right ahead, we cannot find a word character (either the character is not a word character, or it is the end of the string).

What's wrong with your regex

The reason why \b is not suitable is because it requires a word/non-word character to appear after/before it which depends on the immediate context on both sides of \b. When you build a regex dynamically, you do not know which one to use, \B or \b. For your case, you could use '/\bstackoverflow=""\B/', but it would require a smart word/non-word boundary appending. However, there is an easier way: use negative lookarounds.

Solution

(?

See regex demo

The regex contains negative lookarounds instead of word boundaries. The (? lookbehind fails the match if there is a word character before stackoverflow="", and (?!\w) lookahead fails the match if stackoverflow="" is followed by a word character.

What a word shorthand character class \w matches depends if you enable the Unicode modifier /u. Without it, a \w matches just [a-zA-Z0-9_]. You can lay further restrictions using the lookarounds.

Demo

PHP demo:

$re = '/(?

NOTE: If you pass your string as a variable, remember to escape all special characters in it with preg_quote:

$re = '/(?

Here, notice the second argument to preg_quote, which is /, the regex delimiter char.