GKmeena Notes

Python Regex Implementation

The logic of writing a regex (regular expression) formula lies in understanding patterns in the data you want to match. Here’s a step-by-step guide:

Understand the Data

Analyze the format of the data you want to match:

Is it a fixed format (e.g., phone numbers: 123-456-7890)?
Does it have variable parts (e.g., names: John, John Doe)?
Are there any common delimiters (e.g., /, -, .)?

Break Down the Data

Identify individual parts of the pattern:

Characters: Letters, digits, special symbols.
Length: Fixed length or variable length.
Repetition: Is something repeated? (e.g., multiple digits).
Optional Parts: Are any parts optional?

Build the Regex Step-by-Step

Use special characters and quantifiers to describe the data.

Common Regex Components

Character/Pattern	Description	Example
`.`	Matches any single character	`a.b` matches `aab`, `acb`
`\d`	Matches a digit (`0-9`)	`\d{3}` matches `123`
`\w`	Matches a word character (`a-z`, `A-Z`, `0-9`, `_`)	`\w+` matches `hello123`
`\s`	Matches whitespace	`\s` matches a space or tab
`[]`	Matches any character in the set	`[aeiou]` matches any vowel
`^`	Matches the start of the string	`^Hello` matches `Hello world`
`$`	Matches the end of the string	`world$` matches `Hello world`
`*`	Matches 0 or more repetitions	`a*` matches `aaa` or empty
`+`	Matches 1 or more repetitions	`a+` matches `aaa` but not empty
`?`	Matches 0 or 1 repetition	`a?` matches `a` or empty
`{n}`	Matches exactly `n` repetitions	`a{3}` matches `aaa`
`{n,m}`	Matches between `n` and `m` repetitions	`a{1,3}` matches `a`, `aa`, `aaa`
`	`	Logical OR
`()`	Groups patterns	`(abc)+` matches `abcabc`
`\`	Escapes a special character	`\.` matches `.` literally

Examples

1. Phone Number

Match formats like 123-456-7890:
Three digits, a hyphen, three digits, another hyphen, and four digits.

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

2. Email Address

Match formats like user@example.com:
Word characters (\w) or . before @.
A domain name with a .com, .org, etc.

\b\w+(\.\w+)*@\w+\.\w+\b

3. Date

Match formats like DD/MM/YYYY or YYYY-MM-DD:
Two digits, separator (/ or -), two digits, separator, four digits.

(\d{2}[-/]\d{2}[-/]\d{4})
	or 
(\d{4}[-/]\d{2}[-/]\d{2})

Test Your Regex:

Use online tools like Regex101 or Pythex to test your regex.

In Python, you can test it like this:

import re
 
pattern = r'\d{3}-\d{3}-\d{4}'
text = "Call me at 123-456-7890 or 987-654-3210."
matches = re.findall(pattern, text)
print(matches)  # Output: ['123-456-7890', '987-654-3210']

Key tips:

Start small and incrementally refine the regex.
Use \ to escape special characters like ., ?, *, etc.
Always test with different edge cases (e.g., invalid inputs).

By analyzing the structure of the data and applying these components, you can write any regex formula!