What are regular expressions and what they do

What is a regular expression?

Regular expressions are also called pattern expressions. They have a very complete syntax system for writing patterns, and provide a flexible and intuitive string processing method. Regular expressions construct patterns of specific rules and compare them with the input string information to achieve operations such as string matching, search, replacement, and segmentation.

Regular expressions are not a product of PHP itself. They are used in many fields. In addition to being used in perl, C# and Java languages, they are also used in our B/S architecture software development. , regular expressions can be applied to Linux operating systems, front-end JavaScript scripts, back-end scripts PHP and MySQL databases.

Overview of regular expression syntax

Regular expression is a grammatical rule that describes the structure of a string. It is a specific formatting pattern that can match, replace, Intercept the matching string. For users who may have been exposed to DOS before, if it matches all text files in the current folder, they can enter the "dir *.txt" command and all ".txt" files will be listed after pressing the enter key. The "*.txt" here can be understood as a simple regular expression.

PHP supports two styles of regular expression syntax: POSIX and Perl. These two styles of regular expressions are the default styles when PHP is compiled. In PHP 5.3 version, Perl style cannot be disabled. POSIX regular expressions are easier to master, but they are not binary safe.

When are regular expressions used?

All the pattern matching we have done so far has used string functions. We are limited to exact matches, or exact substring matches. If you want to perform some more responsible pattern matching, you should use regular expressions. Regular expressions are difficult to master at first, but they can be extremely useful.

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