Decimal Number Validation Regex

Validate decimal numbers (floating point numbers) with optional minus sign and decimal point using regex pattern.

Pattern Breakdown

^-?\d+(\.\d+)?$

Components

Character Classes

• -? - Optional minus sign
• \d+ - One or more digits
• (\.\d+)? - Optional decimal part:
  • \. - Literal dot (escaped)
  • \d+ - One or more digits after decimal
  • ? - Makes entire group optional

Examples

Valid:

• 123
• 123.45
• -123
• -123.45
• 0.5
• -0.5
• 999.999

Invalid:

• .5 (missing integer part)
• 123. (incomplete decimal)
• 12.34.56 (multiple decimal points)
• +5.5 (plus sign)
• 123.45 (leading space)
• abc (non-numeric)

Implementation

JavaScript

const decimalRegex = /^-?\d+(\.\d+)?$/;
decimalRegex.test('123.45'); // true
decimalRegex.test('-123.45'); // true
decimalRegex.test('123'); // true (integer also valid)
decimalRegex.test('.5'); // false (missing integer part)
decimalRegex.test('123.'); // false (incomplete decimal)

Python

import re
decimal_regex = r'^-?\d+(\.\d+)?$'
bool(re.match(decimal_regex, '123.45'))  # True
bool(re.match(decimal_regex, '-123.45'))  # True
bool(re.match(decimal_regex, '123'))  # True (integer also valid)
bool(re.match(decimal_regex, '.5'))  # False (missing integer part)

Go

decimalRegex := regexp.MustCompile(`^-?\d+(\.\d+)?$`)
decimalRegex.MatchString("123.45") // true
decimalRegex.MatchString("-123.45") // true
decimalRegex.MatchString("123") // true (integer also valid)
decimalRegex.MatchString(".5") // false (missing integer part)

Limitations

1. Accepts integers: Also matches integers (e.g., 123)
2. No scientific notation: Doesn't support 1.23e5 format
3. No leading zeros: Accepts 0123.45 (may want to prevent this)
4. No precision limit: Doesn't limit decimal places
5. String validation: Validates format, not actual float type

When to Use

• Decimal number input validation
• Price/currency input (before formatting)
• Measurement values
• When you need flexible number validation
• Form input validation

For production, consider:

• Requiring decimal part if needed: ^-?\d+\.\d+$
• Supporting scientific notation if needed
• Adding precision limits
• Converting to float type after validation
• Handling locale-specific decimal separators (comma vs dot)