Demystifying Regular Expressions in Python with Examples

 Regex (short for regular expressions) in Python is a powerful and versatile tool for pattern matching and text manipulation. It allows you to search, extract, and manipulate strings based on specific patterns or regular expressions. Regular expressions are a sequence of characters that define a search pattern. They are a standard feature in many programming languages, including Python.

Here's a detailed explanation of regex in Python with examples:

### Basic Syntax:

In Python, you can work with regular expressions using the `re` module, which provides functions for working with regular expressions.

1. **Import the `re` module:**

   import re

2. **Create a regex pattern:**

   To define a regex pattern, you use a string containing special characters that represent various matching rules. For example, the dot (`.`) matches any character, and the asterisk (`*`) matches zero or more occurrences of the preceding character.

### Matching:

You can use regular expressions to find patterns within strings.

import re

text = "Hello, my email is john@example.com"

pattern = r"\b\w+@\w+\.\w+\b"  # Matches email addresses

match = re.search(pattern, text)

if match:

    print("Found:", match.group())

else:

    print("No match found")

In this example:

- `\b` represents a word boundary.

- `\w+` matches one or more word characters (letters, digits, or underscores).

- `@` matches the "@" symbol.

- `\.` matches a period (escaped because it's a special character).

- `re.search()` searches the `text` for the first occurrence of the pattern.

### Extracting:

You can extract matched portions of a string using groups.

import re

text = "Date of birth: 2023-10-02"

pattern = r"(\d{4})-(\d{2})-(\d{2})"  # Matches date in yyyy-mm-dd format

match = re.search(pattern, text)

if match:

    year, month, day = match.groups()

    print(f"Year: {year}, Month: {month}, Day: {day}")

else:

    print("No match found")

In this example, `(\d{4})` captures the year, `(\d{2})` captures the month, and `(\d{2})` captures the day.

### Replacing:

You can use regular expressions to replace specific patterns in a string.

import re

text = "Hello, my name is John. Hello, my name is Alice."

pattern = r"Hello, my name is (\w+)."

new_text = re.sub(pattern, "Hi, my name is \\1.", text)

print(new_text)

In this example, we use `re.sub()` to replace the name with "Hi, my name is {name}".

### Flags:

You can use flags to modify regex behavior. Common flags include `re.IGNORECASE` (case-insensitive) and `re.MULTILINE` (multiline matching).

import re

text = "The quick brown fox\nJumps over the lazy dog"

pattern = r"the (.+?) fox"

match = re.search(pattern, text, re.IGNORECASE | re.MULTILINE)

if match:

    print("Found:", match.group(1))

In this example, we use the `re.IGNORECASE` flag to make the pattern case-insensitive and the `re.MULTILINE` flag to match across multiple lines.