PHP regular expression method to verify specific string length

PHP regular expression is a commonly used string processing method used to verify and match specific character patterns. In some cases, we need to verify whether the length of a string meets the requirements. This article will introduce how to use PHP regular expressions to verify the length of a specific string.

1. Using Qualifiers

In regular expressions, qualifiers are used to specify the minimum and maximum number of repetitions of a pattern match. When verifying a specific string length, you can use the following syntax:

preg_match('/^.{4,8}$/', $str)

where, ^ represents the beginning of the matching string, and $ represents the end of the matching string. . means matching any character, {4,8} means matching any character with the number of repetitions between 4 and 8. This regular expression will match any string between 4 and 8 in length.

2. Using character sets

In regular expressions, character sets are used to match any character in a set of characters. When validating a specific string length, you can define a character set that contains a specific number of characters. For example, if you need to verify a string with a length of exactly 8:

preg_match('/^[a-zA-Z0-9]{8}$/', $str)

where [a-zA-Z0-9] represents a character set containing 26 English letters and 10 digits, {8} represents a match 8 characters. This regular expression will match a string of English letters and numbers of length 8.

3. Using back references

In regular expressions, back references are used to refer to a previously matched pattern. When verifying the length of a specific string, you can use the following syntax:

preg_match('/^(.{8})
$/', $str)

where, means referencing the previously matched .{8} pattern, that is, matching any 8 characters. This regular expression will match a string containing 8 repeated characters, such as "aaaaaaa".

4. Using lookahead and lookbehind

In regular expressions, lookahead and lookbehind are used to search before or after the matching pattern, but do not include the search content. . When verifying the length of a specific string, you can use the following syntax:

preg_match('/^(?=.*[a-z])(?=.*[A-Z])(?=.*d)(?=.*[@$!%*?&])[A-Za-zd@$!%*?&]{8,}$/', $str)

Among them, ?= means lookahead search, ?=.*[a-z] means to search for any lowercase letter. This regular expression will match a string that is at least 8 characters long and contains at least one lowercase letter, one uppercase letter, one number, and one special character.

Summary

This article introduces 4 ways to use PHP regular expressions to verify the length of a specific string. The use of qualifiers, character sets, backreferences, and lookahead and lookbehind can all be used to efficiently verify specific string lengths. In actual development, you can choose a suitable method according to your needs.

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