Preg_replace for substitution in PHP - Step-by-Step Execution

Concept Flow - Preg_replace for substitution

Input string

↓

Define pattern and replacement

↓

Call preg_replace

↓

Search string for pattern matches

↓

Replace matches with replacement

↓

Return new string with substitutions

The flow shows how preg_replace takes an input string, finds parts matching a pattern, replaces them, and returns the new string.

Execution Sample

PHP

<?php
$input = "Hello 123 World 456";
$output = preg_replace('/\d+/', 'NUM', $input);
echo $output;
?>

This code replaces all groups of digits in the input string with 'NUM'.

Execution Table

Step	Action	Pattern Match	Replacement	Resulting String
1	Start with input string	N/A	N/A	Hello 123 World 456
2	Find first match '\d+'	Matches '123'	Replace with 'NUM'	Hello NUM World 456
3	Find second match '\d+'	Matches '456'	Replace with 'NUM'	Hello NUM World NUM
4	No more matches	N/A	N/A	Hello NUM World NUM

💡 No more digit groups found, substitution complete.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
$input	"Hello 123 World 456"	"Hello 123 World 456"	"Hello 123 World 456"	"Hello 123 World 456"
$output	undefined	"Hello NUM World 456"	"Hello NUM World NUM"	"Hello NUM World NUM"

Key Moments - 3 Insights

Why does preg_replace replace all matches, not just the first?

preg_replace by default replaces all matches in the string, as shown in execution_table rows 2 and 3 where both '123' and '456' are replaced.

What does the pattern '/\d+/' mean?

It means 'one or more digits'. This is why '123' and '456' are matched and replaced, as seen in execution_table rows 2 and 3.

Why is the original $input variable unchanged?

preg_replace returns a new string with replacements; it does not modify the original string, as shown in variable_tracker where $input stays the same.

Visual Quiz - 3 Questions

Look at the execution_table, what is the string after the first replacement?

A "Hello NUM World NUM"

B "Hello NUM World 456"

C "Hello 123 World NUM"

D "Hello 123 World 456"

💡 Hint

Check execution_table row 2 under 'Resulting String'.

At which step does preg_replace stop finding matches?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at execution_table row 4 where no more matches are found.

If the pattern was changed to '/World/', what would be the output after replacement?

A "Hello 123 NUM 456"

B "654 MUN 321 olleH"

C Hello 123 NUM 456"

D "Hello 123 NUM 456

💡 Hint

Replacing 'World' with 'NUM' changes only that word; digits remain unchanged.

Concept Snapshot

preg_replace(pattern, replacement, string) searches the string for parts matching the pattern.
It replaces all matches with the replacement text.
Returns a new string; original string stays unchanged.
Patterns use regular expressions, like '/\d+/' for digits.
Useful for flexible text substitutions.

Full Transcript

This visual trace shows how PHP's preg_replace function works step-by-step. Starting with the input string 'Hello 123 World 456', it looks for parts matching the pattern '/\d+/', which means one or more digits. It finds '123' first and replaces it with 'NUM', then finds '456' and replaces it too. The original string remains unchanged, and the function returns a new string with all digit groups replaced by 'NUM'. The process stops when no more matches are found. This helps beginners see how preg_replace scans and replaces all matches, not just the first, and how the pattern controls what is matched.

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