All 7 Regular Expressions in PHP questions
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What is a regular expression, and how do you use PHP's preg_match function to check whether a string matches a specific pattern? What do common regular expression metacharacters like the dot, asterisk, and plus sign mean, and how are they used to build flexible matching patterns? How does preg_split let you break a string apart into an array using a regular expression as the delimiter, and how does this differ from PHP's simpler explode function? What are named capture groups in a regular expression, and how do they improve the readability of extracted match data compared to using only numbered groups? What is catastrophic backtracking in regular expressions, and how can a poorly constructed pattern cause a severe performance problem or even hang a PHP application? When should a developer prefer using a dedicated parser or a simpler string function over a complex regular expression, particularly for structured formats like HTML or JSON?
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How does preg_replace let you find and replace text matching a regular expression pattern, and how do capture groups let you reuse parts of the matched text in the replacement?

Intermediate

Answer

The preg_replace function searches a string for text matching a given pattern and replaces every match with a specified replacement string, and by wrapping specific parts of your pattern in parentheses to create capture groups, you can reference those exact matched portions within the replacement string using a dollar sign followed by the group's number, letting you rearrange or reuse specific pieces of the matched text rather than only ever replacing it entirely.
$formatted = preg_replace('/(\d{3})(\d{3})(\d{4})/', '($1) $2-$3', '5551234567');
echo $formatted;
Real-world example A phone number formatting function uses capture groups within a regular expression to rearrange a plain string of ten digits into a nicely formatted phone number with parentheses and a dash, reusing the originally captured digit groups in a new arrangement.

Common follow-ups: What is the difference between preg_replace and preg_replace_callback?;How do you reference a named capture group instead of a numbered one in the replacement string?

Strings & String Functions;Form Handling & Validation
All 7 Regular Expressions in PHP questions