Preg_replace for substitution in PHP

Introduction

Preg_replace helps you find parts of text that match a pattern and replace them with new text. It is useful when you want to change or clean up text automatically.

You want to replace all phone numbers in a text with a placeholder.

You need to change date formats inside a string.

You want to remove or replace certain words or characters from user input.

You want to add a prefix or suffix to specific words in a sentence.

Syntax

PHP

<?php
mixed preg_replace ( mixed $pattern , mixed $replacement , mixed $subject [, int $limit = -1 [, int &$count ]] )
?>

$pattern is the regular expression pattern to find.

$replacement is the text to replace the matched parts.

$subject is the text where the search and replace happens.

$limit controls how many replacements to make (default is all).

Examples

Replaces all groups of digits with the word 'NUMBER'.

PHP

<?php
$text = "Hello 123, call me at 555-1234.";
$new_text = preg_replace('/\d+/', 'NUMBER', $text);
echo $new_text;
?>

Replaces only the first occurrence of 'def' with 'xyz'.

PHP

<?php
$text = "abc def ghi";
$new_text = preg_replace('/def/', 'xyz', $text, 1);
echo $new_text;
?>

If no match is found, the original text stays the same.

PHP

<?php
$text = "No numbers here!";
$new_text = preg_replace('/\d+/', 'NUM', $text);
echo $new_text;
?>

Sample Program

This program finds phone numbers in the text and replaces them with the word '[PHONE]'. It prints the text before and after the change.

PHP

<?php
// Original text with phone numbers
$original_text = "Contact us at 123-456-7890 or 987-654-3210.";

// Show original text
echo "Before replacement:\n" . $original_text . "\n";

// Replace phone numbers with [PHONE]
$pattern = '/\d{3}-\d{3}-\d{4}/';
$replacement = '[PHONE]';
$modified_text = preg_replace($pattern, $replacement, $original_text);

// Show modified text
echo "After replacement:\n" . $modified_text . "\n";
?>

Important Notes

Time complexity depends on the length of the text and complexity of the pattern, usually O(n) where n is text length.

Space complexity is O(n) because a new string is created with replacements.

Common mistake: forgetting to escape special characters in the pattern (like \d for digits).

Use preg_replace when you need powerful pattern matching. For simple fixed string replacements, str_replace is faster.

Summary

preg_replace finds text matching a pattern and replaces it.

You can control how many replacements happen with the limit parameter.

It is useful for cleaning or formatting text automatically.

Practice

1. What does the PHP function preg_replace do?

easy

A. It splits a string into an array based on a pattern.

B. It counts how many times a pattern appears in a string.

C. It finds text matching a pattern and replaces it with new text.

D. It checks if a string matches a pattern and returns true or false.

Solution

1. Step 1: Understand the purpose of preg_replace

   The function preg_replace searches a string for parts that match a pattern and replaces those parts with new text.

2. Step 2: Compare with other functions

   Counting matches or splitting strings are done by other functions like preg_match_all or preg_split, not preg_replace.

3. Final Answer:

   It finds text matching a pattern and replaces it with new text. -> Option C

4. Quick Check:

   preg_replace replaces matched text [OK]

Hint: Remember: replace means find and change text [OK]

Common Mistakes:

• Confusing preg_replace with preg_match
• Thinking preg_replace counts matches
• Mixing preg_replace with string splitting functions

2. Which of the following is the correct syntax to replace all digits with '#' in PHP using preg_replace?

easy

A. preg_replace('/\D/', '#', $text);

B. preg_replace('/d/', '#', $text);

C. preg_replace('/[0-9]+/', '#', $text);

D. preg_replace('/\d/', '#', $text);

Solution

1. Step 1: Identify the pattern for digits

   The pattern '\d' matches any single digit (0-9). In PHP single-quoted strings, it is written as '/\d/'.

2. Step 2: Check other options

   B uses '/d/', which matches the literal letter 'd'. C uses '/[0-9]+/', which matches one or more consecutive digits, replacing the entire group with a single '#'. D uses '/\D/', which matches non-digits.

3. Final Answer:

   preg_replace('/\d/', '#', $text); -> Option D

4. Quick Check:

   Use '\d' to match digits [OK]

Hint: Use '\d' to match digits in regex [OK]

Common Mistakes:

• Forgetting to escape backslash in PHP strings
• Using 'd' instead of '\d' for digits
• Confusing '\d' with '\D' (non-digits)

3. What will be the output of the following PHP code?

$text = "Hello 123 World";
$result = preg_replace('/\d+/', 'NUM', $text);
echo $result;

medium

A. Hello NUM World

B. Hello 123 World

C. NUM NUM NUM World

D. Hello NUM123 World

Solution

1. Step 1: Understand the regex pattern '/\d+/'

   The pattern '\d+' matches one or more digits in a row. In the string, '123' matches this pattern.

2. Step 2: Apply preg_replace to the string

   The matched digits '123' are replaced by 'NUM'. So the string becomes 'Hello NUM World'.

3. Final Answer:

   Hello NUM World -> Option A

4. Quick Check:

   Digits replaced by 'NUM' = Hello NUM World [OK]

Hint: Look for digit groups replaced by 'NUM' [OK]

Common Mistakes:

• Thinking only single digits replaced separately
• Expecting original digits to remain
• Confusing pattern with non-digit matching

4. Identify the error in this PHP code snippet:

$text = "abc123def";
$result = preg_replace('/[a-z]/', 1, $text, 2);
echo $result;

medium

A. The limit parameter should be the third argument, not the fourth.

B. The replacement value must be a string, not an integer.

C. The pattern should be enclosed in double quotes, not single quotes.

D. preg_replace cannot limit replacements; the code is correct.

Solution

1. Step 1: Check preg_replace argument order

   The arguments are: pattern, replacement, subject, limit. The limit is correctly the fourth argument.

2. Step 2: Check replacement type

   The replacement should be a string. Using integer 1 will be converted to string '1' but is not recommended and can cause confusion.

3. Step 3: Check other options

   Pattern can be in single quotes. preg_replace supports limit parameter.

4. Final Answer:

   The replacement value must be a string, not an integer. -> Option B

5. Quick Check:

   Replacement should be string [OK]

Hint: Always use string for replacement in preg_replace [OK]

Common Mistakes:

• Passing integer instead of string as replacement
• Misplacing the limit parameter
• Thinking preg_replace can't limit replacements

5. You want to replace only the first two occurrences of the word 'apple' with 'orange' in the string 'apple banana apple apple grape'. Which preg_replace call achieves this?

hard

A. preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 2);

B. preg_replace('/apple/', 'orange', 'apple banana apple apple grape');

C. preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 3);

D. preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 1);

Solution

1. Step 1: Understand the limit parameter in preg_replace

   The fourth argument limits how many replacements happen. To replace only first two 'apple', limit should be 2.

2. Step 2: Check each option's limit

   preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 2); uses limit 2, so only first two 'apple' replaced. preg_replace('/apple/', 'orange', 'apple banana apple apple grape'); replaces all. preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 3); replaces first three. preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 1); replaces only first one.

3. Final Answer:

   preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 2); -> Option A

4. Quick Check:

   Limit 2 replaces first two matches [OK]

Hint: Use limit parameter to control replacements count [OK]

Common Mistakes:

• Omitting limit to replace all occurrences
• Setting wrong limit number
• Confusing order of arguments