Regular expression functions (regexp_match, regexp_replace) in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the first match of the pattern 'cat' in the text column.

PostgreSQL

SELECT regexp_match(description, '[1]') FROM animals;

Drag options to blanks, or click blank then click option'

A'fish'

B'dog'

C'bird'

D'cat'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put the pattern inside quotes.

Using a pattern that does not exist in the text.

The regexp_match function returns the first substring matching the given regular expression. Here, we want to find 'cat'.

2fill in blank

medium

Complete the code to replace all digits in the 'phone' column with the character 'X'.

PostgreSQL

SELECT regexp_replace(phone, '[1]', 'X', 'g') FROM contacts;

Drag options to blanks, or click blank then click option'

A'[a-z]'

B'\s'

C'\d'

D'[A-Z]'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a pattern that matches letters instead of digits.

Forgetting the global flag to replace all matches.

The pattern '\d' matches any digit. The 'g' flag means replace all occurrences.

3fill in blank

hard

Fix the error in the code to extract the first word starting with 'a' from the 'text' column.

PostgreSQL

SELECT regexp_match(text, '[1]') FROM documents;

Drag options to blanks, or click blank then click option'

A'a*'

B'\ba\w*\b'

C'[a-z]+'

D'^a'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a pattern that matches partial words or characters only.

Not using word boundaries, causing incorrect matches.

The pattern '\ba\w*\b' matches a whole word starting with 'a'. The '\b' marks word boundaries.

4fill in blank

hard

Fill both blanks to replace all whitespace characters with a single space in the 'notes' column.

PostgreSQL

SELECT regexp_replace(notes, '[1]', '[2]', 'g') FROM logs;

Drag options to blanks, or click blank then click option'

A'\s+'

B' '

C'\d+'

D'_'

Attempts:

3 left

💡 Hint

Common Mistakes

Replacing digits or underscores instead of whitespace.

Not using the global flag to replace all occurrences.

The pattern '\s+' matches one or more whitespace characters. Replacing them with a single space ' ' cleans up the text.

5fill in blank

hard

Fill all three blanks to extract all words starting with a capital letter from the 'sentence' column.

PostgreSQL

SELECT regexp_match(sentence, '[1]', '[2]') FROM texts WHERE regexp_match(sentence, '[3]') IS NOT NULL;

Drag options to blanks, or click blank then click option'

A'\b[A-Z][a-z]*\b'

B'g'

C'i'

D'\d+'

Attempts:

3 left

💡 Hint

Common Mistakes

Using case-insensitive flag 'i' instead of global 'g'.

Using a pattern that matches digits instead of words.

The pattern '\b[A-Z][a-z]*\b' matches words starting with a capital letter. The 'g' flag extracts all matches. The WHERE clause filters rows having such words.

Practice

1. What does the PostgreSQL function regexp_match('abc123', '\\d+') return?

easy

A. An error because of incorrect syntax

B. The string 'abc123' unchanged

C. An empty array because no digits are found

D. An array with the first sequence of digits found: ['123']

Solution

1. Step 1: Understand regexp_match function

   The function searches the input string for the first substring matching the pattern.

2. Step 2: Apply pattern '\\d+' to 'abc123'

   The pattern '\\d+' matches one or more digits. In 'abc123', '123' matches.

3. Final Answer:

   An array with the first sequence of digits found: ['123'] -> Option D

4. Quick Check:

   regexp_match returns first match array [OK]

Hint: regexp_match returns first matching group as array [OK]

Common Mistakes:

• Thinking regexp_match returns the whole string
• Expecting a string instead of an array
• Confusing regexp_match with regexp_replace

2. Which of the following is the correct syntax to replace all digits in 'abc123def' with '#' using regexp_replace in PostgreSQL?

easy

A. regexp_replace('abc123def', '\\d+', '#', 'g')

B. regexp_replace('abc123def', '[0-9]', '#', 'g')

C. regexp_replace('abc123def', '[0-9]+', '#')

D. regexp_replace('abc123def', '\\d', '#')

Solution

1. Step 1: Understand regexp_replace parameters

   regexp_replace(text, pattern, replacement, flags) replaces matching parts. To replace all matches, use flag 'g'.

2. Step 2: Choose pattern and flags to replace all digits

   Pattern '\\d+' matches one or more digits. Flag 'g' ensures all digit sequences replaced, not just first.

3. Final Answer:

   regexp_replace('abc123def', '\\d+', '#', 'g') -> Option A

4. Quick Check:

   Use 'g' flag to replace all matches [OK]

Hint: Use 'g' flag in regexp_replace to replace all matches [OK]

Common Mistakes:

• Omitting 'g' flag replaces only first match
• Using '\\d' without 'g' replaces only one digit
• Confusing pattern syntax with wrong brackets

3. What is the result of this query?

SELECT regexp_match('user_2023_data', 'user_(\\d{4})_data');

medium

A. NULL

B. ['user_2023_data']

C. ['2023']

D. An error due to wrong pattern

Solution

1. Step 1: Analyze the pattern 'user_(\\d{4})_data'

   The pattern looks for 'user_' followed by exactly 4 digits (captured), then '_data'.

2. Step 2: Match pattern against 'user_2023_data'

   The string matches perfectly, capturing '2023' as the group.

3. Final Answer:

   ['2023'] -> Option C

4. Quick Check:

   Captured group returns ['2023'] [OK]

Hint: Captured groups in regexp_match return as array [OK]

Common Mistakes:

• Expecting full string instead of captured group
• Not escaping backslashes properly
• Confusing regexp_match with regexp_replace output

4. Identify the error in this query:

SELECT regexp_replace('abc123def', '\\d+', '#');

It is intended to replace all digit sequences with '#'.

medium

A. regexp_replace cannot replace digits

B. Missing 'g' flag, so only first digit sequence replaced

C. Replacement '#' must be in double quotes

D. Pattern '\\d+' is invalid syntax

Solution

1. Step 1: Understand regexp_replace behavior without flags

   Without 'g' flag, regexp_replace replaces only the first match.

2. Step 2: Check the pattern and replacement

   Pattern '\\d+' is valid and replacement '#' is correct. Only missing 'g' flag causes partial replacement.

3. Final Answer:

   Missing 'g' flag, so only first digit sequence replaced -> Option B

4. Quick Check:

   Use 'g' flag to replace all matches [OK]

Hint: Add 'g' flag to replace all matches in regexp_replace [OK]

Common Mistakes:

• Assuming all matches replaced without 'g'
• Thinking pattern syntax is wrong
• Using single quotes incorrectly for replacement

5. You want to extract the domain name from email addresses like '[e-mail]' using regexp_match. Which query correctly extracts 'example'?

hard

A. SELECT (regexp_match('[e-mail]', '@([\\w]+)\\.com'))[1];

B. SELECT regexp_match('[e-mail]', '@([\\w]+)\\.com');

C. SELECT regexp_replace('[e-mail]', '.*@([\\w]+)\\.com', '\\1');

D. SELECT regexp_replace('[e-mail]', '@([\\w]+)\\.com', '\\1');

Solution

1. Step 1: Understand regexp_match returns array of groups

   regexp_match returns an array with captured groups. To get the domain, access first element [1].

2. Step 2: Check pattern '@([\\w]+)\\.com'

   This pattern captures word characters after '@' and before '.com', which is the domain.

3. Final Answer:

   SELECT (regexp_match('[e-mail]', '@([\\w]+)\\.com'))[1]; -> Option A

4. Quick Check:

   Access first group from regexp_match array [OK]

Hint: Use (regexp_match(...))[1] to get captured group string [OK]

Common Mistakes:

• Not accessing array element from regexp_match
• Using regexp_replace without global flag incorrectly
• Confusing pattern syntax for domain extraction