Regular Expressions (REGEX): A Quick Refresher

If you've ever wanted to find, match, or validate patterns in text, phone numbers, passwords, etc.; then regular expressions are your best friend.

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✅ What is a Regular Expression?

A regular expression, or regex, is a powerful pattern-matching language used to search, extract, or validate text based on specific patterns.

Think of it like a supercharged Ctrl+F, but smarter.

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💡 Why Use Regex?

• Validate emails, phone numbers, passwords, URLs
• Search or replace specific text patterns
• Clean or extract data from messy text
• Apply rules flexibly in a single line

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🛠️ What Does a Regex Look Like?

Here’s a simple one:

/^[A-Z][a-z]+$/

Breakdown:

• ^ → Start of string
• [A-Z] → One uppercase letter
• [a-z]+ → One or more lowercase letters
• $ → End of string

✅ Matches: John, Alice
❌ Doesn’t match: john, ALICE, 123

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📚 Common Regex Symbols

Symbol	Meaning
.	Any character except newline
*	Zero or more of previous item
+	One or more
?	Zero or one
\d	Any digit (0–9)
\w	Any word character (a-z, A-Z, 0-9, _)
[]	Match one of characters inside
()	Grouping
^ / $	Start / end of string

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🧪 Examples

✅ Match a phone number (IL mobile):

/^(\+972|00972|0)5[02345689]\d{7}$/

• The string shall start with (+972 OR 00972 OR 0) followed by a 5 then one of these numbers [02345689] then have any 7 digits.
• Supports local (05X...) and international formats (+9725X...)
• Validates mobile prefixes like 050, 052, 054, etc.

✅ Match an email:

/^[\w.-]+@[a-zA-Z]+\.[a-zA-Z]{2,}$/

Breakdown:

Part	Meaning
^	Anchors the match to the start of the string
[\w.-]+	Matches one or more characters that are: - \w: word characters (letters, digits, underscore) - .: dot (.) - -: dash (-) ⚠️ This is the username before @
@	Matches the literal @ symbol
[a-zA-Z]+	Matches one or more letters, uppercase or lowercase 📍 This is the domain name like gmail, yahoo, etc.
\.	Escaped dot, because . means "any character" in regex. Here we want a literal dot, like in .com or .org
[a-zA-Z]{2,}	Matches 2 or more letters for the domain extension like: com, net, io, co, etc.
$	Anchors the match to the end of the string

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✅ Example Matches:

• [e-mail]
• [e-mail]
• [e-mail]

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🧑‍💻 Which Languages Support Regex?

Regex is supported in almost every major programming language:

Language	Regex Support
JavaScript	✅ Built-in (RegExp)
Python	✅ re module
Java	✅ java.util.regex
PHP	✅ preg_match()
Ruby	✅ Native
Go	✅ regexp package
Rust	✅ regex crate
Bash	✅ grep/sed/awk

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⚠️ Gotchas

• Regex can be powerful, but hard to read.
• Overuse can reduce code readability.
• Always test your regex with tools like regex101.com or RegExr.