★ Dev formats · regex

Regex for 24-hour Time

Matches two-digit 24-hour times with optional seconds. Leap seconds and 24:00 are rejected.

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Try the 24-hour time pattern

Read it in plain English (best-effort)

^start of a line
(?:start a group (not captured)
[01]one character from: '0', '1'
\da digit (0–9)
|or
2the character '2'
[0-3]one character from: 0–3
)end of group
:the character ':'
[0-5]one character from: 0–5
\da digit (0–9)
(?:start a group (not captured)
:the character ':'
[0-5]one character from: 0–5
\da digit (0–9)
)end of group
?the previous is optional
$end of a line

Test text

One sample per line works well with the m flag on anchored patterns.

Waiting for a pattern

The pattern, piece by piece

/^(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d)?$/

Pattern explanation
Part	What it does
`(?:[01]\d\|2[0-3])`	hours 00–23
`:[0-5]\d`	minutes 00–59
`(?::[0-5]\d)?`	optional seconds 00–59

What it accepts and rejects

Every sample below is checked automatically in JavaScript and Python. The tester above is preloaded with all of them, one per line.

Should match
Should match
`00:00`	✓
`09:30`	✓
`23:59`	✓
`12:00:00`	✓
`18:45:59`	✓
`07:05`	✓

Should not match
Should not match
`24:00`	✗
`9:30`	✗
`12:60`	✗
`12:30:60`	✗
`12:30 PM`	✗
`1230`	✗

Copy it into your code

RE2-safeNo lookarounds or backreferences, so the same pattern works in Go's RE2 engine and in Rust's regex crate.

Language

const time24hRe = /^(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d)?$/;

export function isValidTime24h(value) {
  return time24hRe.test(value.trim());
}

When not to use regex for this

If users pick times, use a time input (<input type="time">) instead of free text; the browser enforces the format for you.

Questions engineers actually ask

Does it accept single-digit hours like 9:30?

No. Make the first digit optional with (?:[01]?\d|2[0-3]) if you want to accept them.

Is 24:00 valid?

ISO 8601 allows it to mean end of day, but most systems don't. This pattern rejects it.

What is catastrophic backtracking?

Some patterns, usually nested quantifiers like (a+)+, make a backtracking engine try an exponential number of paths on input that almost matches. The tester stops any run after 300ms, so a bad pattern can't freeze your tab. In production, that same pattern can take down a server (a ReDoS attack).

More dev formats patterns

Or browse the full regex library.

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Fine print

Tools: on the house.
Engineer: by the hour.

The regex was free. Code that doesn't need one is the upgrade: typed schemas, validated once at the edge, trusted everywhere after.

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