Understanding Regex: The Simplest Guide.

Source: Dev.to

What is Regex?

• A pattern‑matching tool used to find, replace, validate, or extract specific patterns in a string.
• Common uses: finding numbers, validating emails, filtering alphabets, removing special characters, extracting dates, phone numbers, IDs, etc.

Regex Syntax in JavaScript

In JavaScript a regex is written between slashes:

/pattern/flags

Example

/[0-9]+/g

Breakdown

Character Sets

The most important part of regex is the character set ([ ]). Whatever you put inside the brackets will match any one of those characters.

Tip: Mastering these five patterns gives you roughly 60 % of what you need to know.

Quantifiers

Quantifiers decide how many times a pattern should repeat.

Anchors

Anchors specify the position of a match within the string.

Example

/^[0-9]+/

→ The string must start with a number.

Flags

Flags come after the closing slash and modify the regex’s behavior.

Example

/hello/gi

Matches: hello, Hello, HELLO, hElLo, etc.

Real‑World Practical Examples

1. Remove Numbers

"50-200 metric tonnes".replace(/[0-9]/g, "");
// Output: "-metric tonnes"

2. Extract Numbers (including hyphen)

"50-200 metric tonnes".match(/[0-9-]+/)[0];
// Output: "50-200"

3. Validate Email

/^[\w.-]+@[\w.-]+\.\w{2,}$/i.test("user@example.com");
// Returns: true

4. Keep Only Letters, Numbers, and Spaces

text.replace(/[^a-zA-Z0-9 ]/g, "");
// Removes all other characters

5. Manual Filtering (without regex)

let result = "";
for (let ch of str) {
  if (!"0123456789".includes(ch)) {
    result += ch;
  }
}

Summary – Key Rules to Remember

• Character set [ ] – what to match
• Quantifiers + * ? – how many times
• Anchors ^ $ – start / end of the string
• Flags g i m s – modify behavior

If you keep these rules in mind, regex becomes easy to use.

What You Can Do with Regex

• Clean strings
• Validate user input
• Extract data
• Perform replacements efficiently

Regex = Pattern + Repetition + Flags
Master this formula and regex will feel simple forever. 🚀