Regex for ISO 8601 Date/Time — Pattern Explained with Examples

This regex validates ISO 8601 date-time strings in the extended format: YYYY-MM-DDTHH:mm:ss with an optional timezone offset (Z or ±HH:mm). This format is the international standard for date and time representation and is widely used in JSON APIs, database timestamps, and data interchange formats like JSON and XML.

The Pattern

/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(\.\d+)?(Z|[+-]\d{2}:\d{2})$/

Pattern Breakdown

Matches

• 2024-01-15T14:30:00Z
• 2024-01-15T14:30:00+05:00
• 2024-01-15T14:30:00.123Z
• 2024-01-15T09:00:00-08:00
• 2024-06-20T23:59:59.999+00:00

Does NOT Match

• 2024-01-15 14:30:00 — space instead of T separator
• 2024-01-15T14:30 — missing seconds
• 01/15/2024T14:30:00Z — wrong date format
• 2024-01-15T24:00:00Z — hour 24 is invalid for ISO 8601 extended time
• 2024-01-15T14:30:00 — no timezone
• 2024-01-15T14:30:00+5:00 — timezone offset missing leading zero on hour

Language Examples

JavaScript

const isoRegex = /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(\.\d+)?(Z|[+-]\d{2}:\d{2})$/;

console.log(isoRegex.test('2024-01-15T14:30:00Z'));         // true
console.log(isoRegex.test('2024-01-15T14:30:00.123+05:00')); // true
console.log(isoRegex.test('2024-01-15 14:30:00Z'));          // false

// Parse ISO string into Date object
const date = new Date('2024-01-15T14:30:00Z');
console.log(date.toISOString());  // 2024-01-15T14:30:00.000Z

Python

import re
from datetime import datetime

iso_regex = r'^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(\.\d+)?(Z|[+-]\d{2}:\d{2})$'

print(bool(re.match(iso_regex, '2024-01-15T14:30:00Z')))              # True
print(bool(re.match(iso_regex, '2024-01-15T14:30:00.123+05:00')))     # True
print(bool(re.match(iso_regex, '2024-01-15 14:30:00Z')))              # False

# Parse ISO string (Python 3.7+)
dt = datetime.fromisoformat('2024-01-15T14:30:00+05:00'.replace('Z', '+00:00'))
print(dt.isoformat())  # 2024-01-15T14:30:00+05:00

Common Pitfalls

1. T separator required — ISO 8601 allows a space in some profiles (like RFC 3339), but the strict standard requires T. Decide which variant your application follows.

2. Z vs offset — Z represents UTC. The alternative +00:00 is equivalent but the regex treats them as separate branches. Make sure your parser handles both.

3. Fractional seconds are optional — The regex allows decimals (.123) but does not enforce a specific precision. Some systems require exactly 3 digits (milliseconds); adjust the quantifier accordingly.

4. Timezone mandatory in this pattern — This regex requires a timezone. If your system allows naive datetimes (no timezone), make the timezone group optional with ?(Z|[+-]\d{2}:\d{2})?$.

Real-World Use Cases

• REST API validation — Ensuring request bodies contain properly formatted timestamps
• Database timestamps — Validating created_at and updated_at fields before insertion
• Log aggregation — Parsing and standardizing timestamps from distributed systems

FAQ