Regex Concepts

2. Literals

2.1 🔤 What are Literals?
1. Literals are normal characters that match exactly as written. Example: catText: The cat is on the roof. Match: cat
2.2 Special Characters
1. Some characters have special meanings in regex. Common special characters:. ^ $ * + ? { } [ ] | ( ) \ If we want to match them as normal text, we must escape them using backslash \. Example: This matches the dollar sign: \$ Normal letters match normally. Special characters need \.

3. ⚡ Special Regex Characters

3.2 Examples
1. Match digits Text: I bought 100 apples. Regex: \d+ Match: 100Match words Text: Hello_123 Regex: \w+ Match: Hello_123Match whitespace Text: Hello World Regex: \s Match: space between Hello and World 📌 Special regex shortcuts help us match common character types quickly.

4. Character Classes

4.1 🔡 What is a Character Class?
1. A character class lets us match one character from a set. Syntax: [abc] This matches: a or b or c

4.3 Ranges
1. We can use ranges inside character classes. - [a-z] matches any lowercase letter. - [A-Z] matches any uppercase letter. - [0-9] matches any digit.
4.4 Negation
1. Inside a character class, ^ means not when written at the beginning. [^0-9] This means: Match anything that is not a digit. 📌 Character classes help us say "match one character from this group."

5. Groups

5.1 🧩 What are Groups?
1. Groups combine multiple characters or patterns together. Groups are useful when we want to: - Reuse part of a pattern - Capture part of matched text - Apply quantifiers to multiple characters - Use alternatives clearly
5.2 Capturing Groups
1. Capturing groups use parentheses: (...) Text matched inside parentheses can be referenced later. Example: (\d{4})([-./])(\d{2})\2(\d{2}) This can match dates like: - 2026-06-15 - 2026.06.15 - 2026/06/15 Here: (\d{4}) captures year ([-./]) captures separator \2 means reuse the second captured group 📌 Simple idea: Capturing groups remember matched parts.
5.3 Capturing Group Example: Website
1. https?://(www\.)?(\w+)(\.\w+)Example text: https://www.google.comPossible captured groups: - \1 captures www. - \2 captures google - \3 captures .com

5.4 Non-Capturing Groups
1. Non-capturing groups use (?:...). They group a pattern but do not store it for back-referencing. Example: (?:Mr|Mrs|Ms)\.?\s[A-Za-z]+ Matches: ✅ Mr Ram ✅ Mrs Smith ✅ Ms Davis But it does not capture the title as a numbered group. 📌 Use (...) when you want to capture. 📌 Use (?:...) when you only want to group.

6. 🔀 Alternation

6.1 🔀 What is Alternation?
1. Alternation means OR. It uses | to match one pattern or another.

6.3 Why Use Groups with Alternation?
1. Without grouping: Mr|Mrs|Ms John This means: Mr OR Mrs OR Ms John Better: (Mr|Mrs|Ms)\sJohn This means: Mr John OR Mrs John OR Ms John 📌 Simple idea: Use | for OR. Use groups to control what OR applies to.

7. 🔢 Quantifiers

7.1 🔢 What are Quantifiers?
1. Quantifiers tell regex how many times the previous pattern should appear. Sample text: I bought 100 apples and 50 oranges.

7.3 Useful Quantifier Examples
1. Match a 10-digit phone number: \d{10} Matches: 9841000000 Match one or more letters: [a-zA-Z]+Match optional s: apples? Matches: apple, apples 📌 Quantifiers control "how many."

8. 🍽️ Greedy vs Lazy Quantifiers

8.1 🍽️ Greedy Quantifiers
1. A greedy quantifier tries to match as much text as possible. Sample text: <div>Content</div><div>More</div>Pattern: <div>.*</div>Match: <div>Content</div><div>More</div> It matches too much because .* is greedy.
8.2 🐜 Lazy Quantifiers
1. A lazy quantifier tries to match as little text as possible. We make a quantifier lazy by adding ?. Pattern: <div>.*?</div> Matches: <div>Content</div> <div>More</div>

10. 👁️➡️ Lookahead

10.1 Positive Lookahead
1. Syntax: (?=...) It checks if a pattern appears after the current position. Example: Text: Weight: 50kg and 20m Regex: \d+(?=kg) Match: 50 It matches 50 only because it is followed by kg.
10.2 Negative Lookahead
1. Syntax: (?!...) It checks that a pattern does not appear after the current position. Example: Text: Weight: 50kg, Count: 20 items Regex: \d+\b(?!kg) Possible match: 20 📌 Lookahead checks what comes after, but does not include it in the match.

11. 👁️⬅️ Lookbehind

11.1 Positive Lookbehind
1. Syntax: (?<=...) It checks if a pattern appears before the current position. Example: Text: Price: $100 or Rs 15000 Regex: (?<=\$)\d+ Match: 100 It matches 100 only because it is preceded by $.
11.2 Negative Lookbehind
1. Syntax: (?<!...) It checks that a pattern does not appear before the current position. Example: Text: Price: $100, Quantity: 20 Regex: (?<!\$)\b\d+\b Possible match: 20 📌 Lookbehind checks what comes before, but does not include it in the match.

12. ⚓ Anchors

12.1 ⚓ What are Anchors?
1. Anchors are used to match the start or end of a string.

12.2 Anchor Examples
1. Sample text: The cat is on the roof.

13. 🎯 Practical Regex Examples

13.1 Match Email-like Text
1. [\w.-]+@[\w.-]+\.\w+Example match: [email protected]
13.2 Match Nepali Phone Number Style
1. 9[78]\d{8}Example match: 9841000000
13.3 Match Date Format YYYY-MM-DD
1. \d{4}-\d{2}-\d{2}Example match: 2026-07-16
13.4 Match Price
1. \$\d+Example match: $100