Regex Tutorial - How to write Regular Expressions

A regular expression (regex) is a sequence of characters that defines a search pattern. It is mainly used for pattern matching in strings, such as finding, replacing, or validating text. Regex is supported in almost every programming language, including Python, Java, C++ and JavaScript.

Below image shows an example of a regular expression and explains its parts, helping you understand how filenames or patterns can be matched effectively.

This regex checks if a filename is valid, allowing letters, numbers, underscore, hyphens and ends with .jpg, .png or .gif. Example matches: file123.jpg, my-photo.png, logo_1.gif.

Examples: Match a Filename Ending with .jpg, .png, or .gif

Importance of Regular Expression

1. Efficient Pattern Matching: Quickly search for specific patterns in text or data without manual checking.
2. Data Validation: Validate inputs like email addresses, phone numbers, URLs, and passwords.
3. Text Manipulation: Perform search-and-replace operations across files or datasets effectively.
4. Automation in Analytics and Tools: Used in tools like Google Analytics for URL matching and filtering.
5. Cross-Platform Support: Works across programming languages and editors such as Python, Java, Sublime Text, Notepad++, and Microsoft Word.

Common Elements Used in Regular Expressions

Regular expressions are built using special symbols and characters. Below are the most commonly used regex elements explained with simple examples.

1. Repeaters (  *, +, and { } ): Repeaters specify how many times the preceding character or group should appear.

2. Asterisk symbol (*): Matches the preceding character 0 or more times.

Example: The regular expression ab*c will give ac, abc, abbc, abbbc….and so on

3. The Plus symbol (+): Matches the preceding character 1 or more times.

Example: The regular expression ab+c will give abc, abbc, abbbc, … and so on.

4. The curly braces { … }: Defines an exact or range of repetitions.

Example: {{2}: exactly 2 times
{min,}: at least min times
{min,max}: between min and max times

5. Wildcard (.): Matches any single character except a newline.

Example: Regular expression .* will tell the computer that any character can be used any number of times.

6. Optional character (?): Matches 0 or 1 occurrence of the preceding character.

Example: docx? matches doc and docx

7. The caret ( ^ ) symbol: Ensures the match starts at the beginning of the string.

Example : ^\d{3} matches 901 in 901-333

8.  The dollar ( $ ) symbol: Ensures the match ends at the end of the string.

Example: \d{3}$ matches 333 in 901-333

9. Character Classes: Match specific types of characters: 

\s: whitespace
\S: non-whitespace
\d: digit
\D: non-digit
\w: word character (letters, digits, _)
\W: non-word character
\b: word boundary

Example: [abc] matches a, b, or c

10. Negated Character Class ([^ ]): Matches characters not listed in the brackets.

Example : [^abc] -> matches any character except a, b, c