Regular Expression

Regular Expressions

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Prerequisites

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1. What is a Regular Expressions

A Regular Expression (Regex) is a pattern used to match, search, or extract text. Instead of writing complex parsing logic, you describe patterns:

"\\d+" → one or more digits

2. Why Use Regex?

• extract data (emails, numbers, logs)
• validate input (passwords, IDs)
• search patterns
• text processing

Use when:

• pattern is complex
• structure is unknown
• flexibility needed

Avoid when:

• simple parsing
• performance critical
• tight loops

The Regex processing is expensive

3. How to Use Regex?

✔️ Literals

abc   → matches "abc"

✔️ Metacharacters

Symbol	Meaning
.	any character
\d	digit
\w	word character
\s	whitespace

✔️ Quantifiers

Pattern	Meaning
*	0 or more
+	1 or more
?	0 or 1
{n}	exactly n
{n,m}	between n and m

✔️ Examples

\d+      → 123, 456
\w+      → hello, test123
a.*b     → a___b

✔️ Character Classes

[abc]    → a or b or c
[^abc]   → NOT a, b, c
[a-z]    → lowercase letters
[A-Z]    → uppercase letters
[0-9]    → digits

✔️ Anchors

^   → start of string
$   → end of string

Example

^abc   → starts with "abc"
xyz$   → ends with "xyz"

4. Groups and Captures

✔️ Grouping

(abc)

✔️ Capture Example

(\w+)=(\d+)

Matches:

age=30

• Group 1 → age
• Group 2 → 30

5. Miscellaneous

✔️ Alternation

cat|dog

matches “cat” or “dog”

✔️ Greedy (default)

a.*b

matches longest possible

a123b456b → a [123b456] b

✔️ Lazy

a.*?b

matches shortest possible

a123b456b → a [123] b

meta characters

.  *  +  ?  ^  $  ( )  [ ]  { }  |  \

escape(‘\’) must need

\w+ → \\w+

6. Related functions in C++

Function	Description
regex_match	full match
regex_search	partial match
regex_replace	replace
regex_iterator	iterate matches

7. Useful Patterns

✔️ Email

\w+@\w+\.\w+

\\w+@\\w+\\.\\w+ (c++)

✔️ Number

\d+

✔️ Date (YYYY-MM-DD)

\d{4}-\d{2}-\d{2}

✔️ Phone

\d{3}-\d{3}-\d{4}