Preg_replace for substitution in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the preg_replace function do in PHP?

preg_replace searches a string for matches to a regular expression pattern and replaces them with a specified replacement string.

beginner

What are the three main parameters of preg_replace?

• pattern: The regex pattern to search for.
• replacement: The string to replace matches with.
• subject: The input string to search and replace in.

beginner

How do you use preg_replace to replace all digits in a string with a dash (-)?

preg_replace('/\d/', '-', 'Phone: 123-456'); // Result: 'Phone: --- ---'

beginner

What happens if no matches are found when using preg_replace?

The original string is returned unchanged because there is nothing to replace.

intermediate

Can preg_replace use backreferences in the replacement string? Give an example.

<p>Yes. Backreferences let you reuse parts of the matched text in the replacement.</p><pre>preg_replace('/(\w+) (\w+)/', '$2 $1', 'John Doe'); // Result: 'Doe John'</pre>

What does preg_replace('/cat/', 'dog', 'catapult') return?

A catapult

B catdog

C dog

D dogapult

The pattern 'cat' is found at the start of 'catapult' and replaced with 'dog', resulting in 'dogapult'.

Which delimiter is used in the pattern /\d+/ in preg_replace?

A #

B /

C ~

D %

The slashes / are the delimiters that mark the start and end of the regex pattern.

What will preg_replace('/(\w+) (\w+)/', '$2 $1', 'Hello World') output?

A $2 $1

B Hello World

C World Hello

D Hello

The two words are swapped using backreferences $2 and $1 in the replacement string.

If the pattern does not match anything in the subject, what does preg_replace return?

A The original subject string

B NULL

C An error

D An empty string

No matches means no replacements, so the original string is returned unchanged.

Which of these is a valid use of preg_replace to remove all vowels from a string?

A preg_replace('/[aeiou]/i', '', 'Hello')

B preg_replace('aeiou', '', 'Hello')

C preg_replace('/aeiou/', '', 'Hello')

D preg_replace('/[aeiou]/', ' ', 'Hello')

Option A uses a regex pattern with character class and case-insensitive flag i to remove vowels.

Explain how preg_replace works for substituting text in a string.

Think about how you find and replace words using a pattern.

Describe how backreferences can be used in the replacement string of preg_replace.

Imagine swapping parts of a matched phrase.

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