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Tala Amm
Tala Amm

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Regular Expressions (REGEX): A Quick Refresher

#javascript #beginners #programming #regex

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

✅ 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.

💡 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

🛠️ What Does a Regex Look Like?

Here’s a simple one:

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

📚 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

🧪 Examples

✅ Match a phone number (IL mobile): 

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

✅ Example Matches:

  • tala.dev@gmail.com
  • example-world_123@dev.to.org
  • a@b.co

🧑‍💻 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

⚠️ 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.

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