Why do random UUIDs keep a 4 in the same position?

UUID Generator produces UUID v4 values with a deliberate structure: the third hyphen-separated group starts with 4, and the fourth starts with 8, 9, a or b. Those positions carry version and variant information. Check the group lengths and these markers when inspecting a pasted fixture, but remember that a matching shape cannot prove randomness or uniqueness.

Open UUID Generator

Find the two constrained positions

If several generated identifiers have a 4 in the same place, that is expected. Read the identifier as five groups separated by hyphens. The third group begins with 4 because this tool generates version 4. The first character of the fourth group is restricted to 8, 9, a or b by the variant bits. Repeating either marker does not itself indicate a repeated identifier.

For example, 123e4567-e89b-42d3-a456-426614174000 illustrates the layout. Its third group is 42d3, so the version character is 4. Its fourth group is a456, so the variant character is a. This is a reading example, not a newly generated value to reuse across records that need different IDs.

See how the fallback preserves randomness around the markers

The implementation first uses the browser's crypto.randomUUID function when it is available. Otherwise, it requests 16 bytes through crypto.getRandomValues. In that fallback, the app changes only the bits needed for the version and variant, then converts the bytes to hexadecimal and inserts the hyphens.

The version operation keeps the lower four bits of byte 6 and sets the upper four to the hexadecimal digit 4. Byte numbering here starts at zero. The variant operation keeps the lower six bits of byte 8 and sets its upper two bits to binary 10. Combined with the remaining bits, that makes the first hexadecimal digit of the fourth group 8, 9, a or b.

You therefore should not expect every character to vary freely. The fixed 4 and limited variant character are part of the intended output. The fallback formats the other bytes as two-digit hexadecimal values, retaining leading zeroes so the groups keep their full lengths.

Check a pasted fixture one group at a time

When an identifier in a test fixture looks wrong, compare its text with the generator's output shape. Expect 36 characters: 32 lowercase hexadecimal digits and four hyphens. The five group lengths are 8, 4, 4, 4 and 12. Letters in this app's output run from a through f; a character such as g does not belong in that shape.

Next, inspect the third group's first character and the fourth group's first character. Changing the example's 42d3 to 52d3 keeps the same total length but no longer matches this tool's v4 format. Changing a456 to c456 also preserves the length but falls outside the variant characters checked by the app's source test. Length alone misses both mistakes.

Make this inspection in your destination file or editor. UUID Generator has a read-only output field and no input validator for existing identifiers. These checks describe the shape this app emits; they do not establish which other UUID representations or versions your destination accepts.

Keep format checks separate from identity checks

A correctly shaped string does not reveal how it was produced. Someone could type the example above by hand and satisfy the same character checks. Passing them also cannot show whether that exact identifier already exists in your project. The app explicitly describes uniqueness as probabilistic and cannot prove global uniqueness.

Use the structure check to catch a damaged paste or an unsuitable fixture, then check the destination's requirements and existing records separately. If a fixture's ID is referenced elsewhere, keep those references consistent when replacing it. The generator creates values; it does not inspect your files, validate relationships or reconcile IDs already in use.

AI-assisted ForgeHub editorial. App capabilities were checked against its implementation; this guide is not an independent review.