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楊東霖
楊東霖

Posted on • Originally published at devtoolkit.cc

The Complete Guide to Regular Expressions in 2026

#regex #webdev #programming #tutorial

Regular expressions — regex — are one of the most powerful tools in a developer's toolkit. They're also one of the most intimidating. But regex is genuinely worth learning.

The Basic Building Blocks

Character Classes 

/[abc]/     // matches 'a', 'b', or 'c'
/[0-9]/     // matches any digit (same as \d)
/[a-z]/     // matches any lowercase letter
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Quantifiers 

/\d+/       // 1 or more digits
/\w{3}/     // exactly 3 word characters
/https?/    // matches "http" or "https"
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Real-World Examples

Email Validation 

/^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/
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Password Strength 

// At least 8 chars, 1 uppercase, 1 lowercase, 1 number, 1 special char
/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$/
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Lookahead and Lookbehind

Match something based on what comes before or after it:

/\d+(?=\$)/    // match digits only when followed by $
// "100 dollars and 50$" → matches "50"
/(?<=\$)\d+/     // match digits only when preceded by $
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The best developers use regex as a precision tool — start simple, build up complexity, and test every pattern before deploying it.

Try our Regex Tester to experiment with patterns instantly.

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