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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? 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? 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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What do common regular expression metacharacters like the dot, asterisk, and plus sign mean, and how are they used to build flexible matching patterns?

Beginner

Answer

The dot matches any single character except a newline, the asterisk means the preceding character or group can appear zero or more times, and the plus sign means the preceding character or group must appear one or more times, and combining these basic building blocks together, along with character classes and other metacharacters, lets you construct patterns that flexibly match a wide range of possible text variations rather than only a single exact string.
preg_match('/colou?r/', 'color');
preg_match('/colou?r/', 'colour');
// Both match, since the question mark makes the u optional
Real-world example A text search feature uses a pattern with an optional character to correctly match both the American and British spelling of a word, accommodating both variations with a single flexible pattern rather than needing two separate checks.

Common follow-ups: What is the difference between the asterisk and the plus sign quantifiers?;How do you match a literal dot character rather than having it match any character?

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