Regex Tester — Developer Code Samples

Regular expressions are powerful pattern-matching tools built into every major language. Python's re module, JavaScript's RegExp, and grep/sed provide regex support with slightly different syntax. These examples cover common use cases including validation, extraction, and substitution.

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Parameters

Parameter	Type	Required	Description
pattern	str	Yes	Regular expression pattern string
text	str	Yes	Input text to test the pattern against
flags	str	No	Regex flags: i (ignore case), m (multiline), s (dotall)

Returns: List of matches (findall), single match object (match/search), or substituted string (sub)

Code Examples

import re

# Test if a pattern matches
pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
email = "[email protected]"
is_match = bool(re.match(pattern, email))
print(f"Valid email: {is_match}")  # True

# Find all matches
text = "Prices: $10.99, $5.50, $100.00"
prices = re.findall(r'\$([\d.]+)', text)
print(prices)  # ['10.99', '5.50', '100.00']

# Extract named capture groups
log_pattern = r'(?P<date>\d{4}-\d{2}-\d{2}) (?P<level>\w+) (?P<message>.+)'
log_line = "2024-01-15 ERROR Database connection failed"
m = re.match(log_pattern, log_line)
if m:
    print(m.group('date'))     # 2024-01-15
    print(m.group('level'))    # ERROR
    print(m.group('message'))  # Database connection failed

# Substitution
text = "Hello   World   Foo"
cleaned = re.sub(r'\s+', ' ', text)
print(cleaned)  # Hello World Foo

# Case-insensitive search
hits = re.findall(r'python', "I use Python and python", re.IGNORECASE)
print(hits)  # ['Python', 'python']

# Split on multiple delimiters
parts = re.split(r'[,;|]', "one,two;three|four")
print(parts)  # ['one', 'two', 'three', 'four']

// Test if pattern matches
const emailPattern = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
console.log(emailPattern.test("[email protected]"));  // true
console.log(emailPattern.test("not-an-email"));       // false

// Find all matches
const text = "Prices: $10.99, $5.50, $100.00";
const prices = [...text.matchAll(/\$([\d.]+)/g)].map(m => m[1]);
console.log(prices);  // ['10.99', '5.50', '100.00']

// Named capture groups
const logLine = "2024-01-15 ERROR Database connection failed";
const logPattern = /(?<date>\d{4}-\d{2}-\d{2}) (?<level>\w+) (?<message>.+)/;
const match = logLine.match(logPattern);
if (match) {
  console.log(match.groups.date);     // 2024-01-15
  console.log(match.groups.level);    // ERROR
  console.log(match.groups.message);  // Database connection failed
}

// Replace with regex
const cleaned = "Hello   World".replace(/\s+/g, ' ');
console.log(cleaned);  // Hello World

// Split on multiple delimiters
const parts = "one,two;three|four".split(/[,;|]/);
console.log(parts);  // ['one', 'two', 'three', 'four']

// Replace with function
const result = "hello world".replace(/(\w+)/g, w => w.toUpperCase());
console.log(result);  // HELLO WORLD

# Test if string matches pattern (grep)
echo "[email protected]" | grep -E '^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'

# Find all matches in a file
grep -oE '\$[0-9]+\.[0-9]+' prices.txt

# Substitution with sed
echo "Hello   World" | sed 's/[[:space:]]\+/ /g'

# Case-insensitive grep
grep -i "error" logfile.txt

# Extract named groups with Python
python3 -c "
import re, sys
m = re.search(r'(\d{4}-\d{2}-\d{2})', sys.argv[1])
print(m.group(1) if m else 'no match')
" '2024-01-15 Log entry'

# Replace with sed (global)
sed -i 's/old_text/new_text/g' myfile.txt

Other Tool APIs

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View all API docs →