How Can I Replace Whole Words in a String Using Regular Expressions?

Finding and Replacing Whole Word Matches in a String

When working with strings, it's common to need to replace specific words or phrases. However, simply using a string replacement function like str_replace() can lead to unexpected results, as it replaces all occurrences of the pattern regardless of its position within a word.

To address this, regular expressions can be employed to match only whole word instances of a pattern. The key to this is the word boundary metacharacter, denoted by "b".

Using Regular Expressions

To replace only whole word matches, use the following regular expression pattern:

/\bHello\b/

Here's a breakdown of the pattern:

• "/" (delimiter): Indicates the start and end of the regular expression.
• "b" (word boundary): Matches the beginning or end of a word.
• "Hello" (pattern): The word you want to match.
• "/" (delimiter): Indicates the end of the regular expression.

Example Implementation

Consider the following PHP code:

<?php $text = "Hello hellol hello, Helloz";
$newtext = preg_replace('/\bHello\b/', 'NEW', $text);
echo $newtext;
?>

Explanation:

• The preg_replace() function is used to perform the regular expression replacement.
• The regular expression /bHellob/ is used to find and replace whole word matches of "Hello" with "NEW".
• The result is stored in $newtext.

Output:

NEW hello1 hello, Helloz

Unicode Considerations

If your text contains Unicode characters, you may need to add the "u" modifier to the regular expression to handle non-latin characters correctly:

$newtext = preg_replace('/\bHello\b/u', 'NEW', $text);

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