Regular expression matching (~ operator) in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the ~ operator do in PostgreSQL?

The ~ operator checks if a string matches a regular expression pattern. It returns true if the pattern matches, false otherwise.

beginner

How do you write a query to find rows where the column name contains only digits using ~?

Use WHERE name ~ '^\d+$'. This means the entire name must be digits from start (^) to end ($).

intermediate

What is the difference between ~ and ~* operators in PostgreSQL?

~ is case-sensitive regex match, while ~* is case-insensitive regex match.

intermediate

Write a PostgreSQL query using ~ to find emails ending with @example.com.

SELECT * FROM table WHERE email ~ '@example\.com$';
The \. escapes the dot, and $ means end of string.

intermediate

Can the ~ operator be used to check for multiple patterns at once?

No, ~ matches one regex pattern at a time. To check multiple patterns, combine conditions with OR or use regex alternation like pattern1|pattern2.

What does the PostgreSQL operator ~ do?

A Concatenates two strings

B Performs a case-insensitive string comparison

C Checks if a string contains only letters

D Matches a string against a regular expression pattern

The ~ operator matches a string against a regex pattern and returns true if it matches.

Which operator would you use for case-insensitive regex matching in PostgreSQL?

A ~*

B ~

C !~

D ILIKE

~* is the case-insensitive regex match operator in PostgreSQL.

What does the regex pattern ^\d+$ mean when used with ~?

A String contains at least one digit

B String contains only digits from start to end

C String ends with a digit

D String starts with a digit

The pattern means the entire string (from ^ start to $ end) consists only of digits (\d+).

How do you escape a dot (.) in a regex pattern for PostgreSQL?

A \.

B .

C \d

D \*

A dot means any character in regex, so to match a literal dot, you escape it with a backslash: \.

Can the ~ operator match multiple patterns separated by commas?

A Yes, it matches all patterns automatically

B Yes, but only if patterns are in quotes

C No, it matches only one regex pattern at a time

D No, it only matches numeric patterns

~ matches one regex pattern at a time. To match multiple patterns, use alternation or multiple conditions.

Explain how the ~ operator works in PostgreSQL and give an example query.

Think about how you check if a string fits a pattern using regex.

Describe the difference between ~ and ~* operators and when to use each.

Consider if letter case matters in your pattern matching.

Practice

1. What does the PostgreSQL operator ~ do when used in a query like column_name ~ 'pattern'?

easy

A. Returns the length of the column value

B. Replaces the column value with the pattern

C. Checks if the column value matches the regular expression pattern

D. Counts the number of times the pattern appears in the column

Solution

1. Step 1: Understand the ~ operator

   The ~ operator in PostgreSQL is used to check if a string matches a regular expression pattern.

2. Step 2: Apply to the query context

   When used as column_name ~ 'pattern', it returns true if the column's value matches the pattern, otherwise false.

3. Final Answer:

   Checks if the column value matches the regular expression pattern -> Option C

4. Quick Check:

   ~ means match pattern [OK]

Hint: Remember: ~ means 'matches pattern' in PostgreSQL [OK]

Common Mistakes:

• Thinking ~ replaces or modifies data
• Confusing ~ with counting occurrences
• Assuming ~ returns string length

2. Which of the following is the correct syntax to find rows where the column email contains the domain example.com using the ~ operator?

easy

A. SELECT * FROM users WHERE email ~ 'example.com';

B. SELECT * FROM users WHERE email ~ '@example\.com$';

C. SELECT * FROM users WHERE email LIKE '%example.com%';

D. SELECT * FROM users WHERE email = '~example.com';

Solution

1. Step 1: Understand the pattern for matching domain

   To match emails ending with '@example.com', the regex should be @example\.com$ where \. escapes the literal dot and $ means end of string.

2. Step 2: Check the syntax of the ~ operator usage

   The correct syntax is email ~ '@example\.com$' to find emails ending with that domain.

3. Final Answer:

   SELECT * FROM users WHERE email ~ '@example\.com$'; -> Option B

4. Quick Check:

   Regex match with ~ and end anchor [OK]

Hint: Use $ to anchor regex at string end with ~ operator [OK]

Common Mistakes:

• Using LIKE instead of ~ for regex matching
• Missing anchors like $ for exact domain match
• Using = operator with regex pattern

3. Given the table products with a column code containing values: 'A123', 'B234', 'A345', 'C456', what will the query SELECT code FROM products WHERE code ~ '^A'; return?

medium

A. A123, A345

B. B234, C456

C. All codes

D. No rows

Solution

1. Step 1: Understand the regex pattern ^A

   The caret ^ means the string must start with 'A'.

2. Step 2: Apply pattern to each code

   Codes starting with 'A' are 'A123' and 'A345'.

3. Final Answer:

   A123, A345 -> Option A

4. Quick Check:

   ^ means start with 'A' [OK]

Hint: ^ anchors regex to start of string [OK]

Common Mistakes:

• Confusing ^ with end anchor $
• Expecting all rows to match
• Ignoring regex anchors

4. You wrote the query SELECT * FROM users WHERE username ~ '*'; but it returns an error. What is the most likely cause?

medium

A. The regex pattern is missing escape characters for special symbols

B. The ~ operator requires the pattern to be enclosed in double quotes

C. The regex pattern is invalid because it lacks anchors

D. The pattern is correct but the column contains NULL values causing error

Solution

1. Step 1: Check regex pattern syntax

   The pattern * is invalid because the * quantifier requires a preceding atom (e.g., a character or group).

2. Step 2: Identify common error with special characters

   Regex special symbols like *, +, ? need escape characters (e.g., \* in SQL string) when used literally.

3. Final Answer:

   The regex pattern is missing escape characters for special symbols -> Option A

4. Quick Check:

   Escape special regex chars in SQL strings [OK]

Hint: Escape special regex characters properly in SQL strings [OK]

Common Mistakes:

• Using single quotes incorrectly for regex
• Assuming ~ needs double quotes
• Ignoring NULL values causing errors

5. You want to select all rows from orders where the order_id starts with a letter followed by exactly three digits. Which query using the ~ operator is correct?

hard

A. SELECT * FROM orders WHERE order_id ~ '^\w{4}$';

B. SELECT * FROM orders WHERE order_id ~ '^[0-9]{3}[A-Za-z]$';

C. SELECT * FROM orders WHERE order_id ~ '[A-Za-z][0-9]{3}';

D. SELECT * FROM orders WHERE order_id ~ '^[A-Za-z][0-9]{3}$';

Solution

1. Step 1: Understand the pattern requirements

   The order_id must start with a letter [A-Za-z] followed by exactly three digits [0-9]{3}, and nothing else.

2. Step 2: Check regex anchors and pattern

   Using ^ and $ anchors ensures the entire string matches. '^[A-Za-z][0-9]{3}$' matches a letter then three digits exactly.

3. Final Answer:

   SELECT * FROM orders WHERE order_id ~ '^[A-Za-z][0-9]{3}$'; -> Option D

4. Quick Check:

   ^ and $ anchor full string match [OK]

Hint: Use ^ and $ to match entire string pattern [OK]

Common Mistakes:

• Swapping letter and digit order
• Missing anchors causing partial matches
• Using \w which includes digits and letters