PHP preg_quote to escape regex characters
Jul 13, 2025 am 01:54 AM
When processing regular expressions, when you need to insert user input or external data into the regular as a literal string, the preg_quote() function needs to be used to escape special characters. 1. When a variable contains special characters in a regular (such as ., *, , ?) and is spliced into a regular expression, it will cause abnormal matching behavior; 2. The first parameter of preg_quote() is a string to be escaped, and the second parameter is used to specify the regular separator to ensure that the separator itself is also escaped; 3. Common misunderstandings include abuse of preg_quote(), ignoring the delimiter parameters, and mistakenly thinking that it can solve all security problems; 4. In practical applications, it is recommended to always use preg_quote() to process dynamic content and develop standard writing methods: $pattern = '/' . preg_quote($search_term, '/') . '/i';.
When dealing with regular expressions, we often need to insert user input or external data into the regular as literal strings. At this time, if the string contains regular special characters (such as . , * , , ? , etc.), which will lead to errors in matching behavior. To solve this problem, PHP provides the preg_quote() function to automatically escape these special characters.
When do you need to use preg_quote?
When you splice a variable directly into a regular expression, for example:
$keyword = "example.com"; $pattern = "/$keyword/i";
If $keyword contains symbols like . or ? , they are treated as regular operators instead of ordinary characters. This may cause the matching results to not meet expectations.
At this time, you should use preg_quote() to ensure that all special characters are escaped and become ordinary characters.
Basic usage of preg_quote
The function prototype is as follows:
preg_quote(string $string, ?string $delimiter = null): string
- The first parameter is the string to escape.
- The second parameter is an optional regular separator. If you use a separator like
/and#, passing it in can allowpreg_quote()to escape this separator at the same time.
To give a simple example:
$keyword = "test.example?com"; $escaped = preg_quote($keyword, '/'); echo $escaped; // Output: test\.example\?com
In this way, there will be no problem with splicing into the regular:
$pattern = '/' . preg_quote($keyword, '/') . '/i';
Common misunderstandings and precautions
Although preg_quote() is useful, some points are easily overlooked:
- Don't abuse : If you are just doing simple string replacement and do not need regular functions, you can use non-regular functions such as
strpos()andstr_replace(). - Don't miss the delimiter parameters : If your regular uses a delimiter like
/or#, remember to add a second parameter, otherwise it may cause an error because the same character appears in the string. - Not "absolutely safe" : it is only responsible for escaping regular meta characters and does not prevent injection or other types of security issues.
Suggestions in practical applications
- When building dynamic regular expressions, always use
preg_quote()to process variable content. - If you are not sure if you need to escape, you would rather add it than remove it.
- Develop habits when splicing regularities:
$pattern = '/' . preg_quote($search_term, '/') . '/i';
This method can avoid many hidden problems.
Basically that's it. Just remember one sentence: All variable contents spliced into the regular must be passed through preg_quote first .
The above is the detailed content of PHP preg_quote to escape regex characters. For more information, please follow other related articles on the PHP Chinese website!
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