[djmakinera]

How do find a separate number of characters?

Incorrect pattern because it matches all characters!

Spoiler:

={1,3}?

Non-Greedy n through m
{x,y} = match the preceding element x - y times.
1 is the minimum number of characters to match.
3 is the maximum number of consecutive characters to match.
? = nongreedy

Spoiler:

= (1 char)
== (2 char)
=== (3 char)
====
=====
====== (6 char)

[Jiaz]

why the ?
so more than = are not allowed?

^={1,3}(?![=]+)

[djmakinera]

Because 3 characters are unified everywhere. But that I could make a mistake anywhere, now I can correct it quickly.

This pattern is also correct:
For example: The minimum / maximum number of characters is 3. In this case.

^(=){3,3}$