This Commented CUE demonstrates how to make sure a string has a minimum length, or a maximum length, or both.

This CUE shows string fields being constrained by their lengths as expressed in bytes, and also in runes, as introduced by the Go programming language.

example.cue
package example

import "strings"

// 2 examples that use only single-byte, ASCII characters
ascii_constrain_runes: "only ASCII characters" // 21 runes composed of 21 bytes
// enforce that the field contains between 2 and 8 runes
ascii_constrain_runes: strings.MinRunes(2) & strings.MaxRunes(8)

ascii_constrain_bytes: {
	// set up an alias to allow the following CUE to refer to it
	let content = "only ASCII characters" // 21 runes composed of 21 bytes

	// this embedded scalar ultimately sets the value of the containing field,
	// "ascii_constrain_bytes"
	content

	// enforce that "content" contains between 50 and 100 bytes
	#bytes: len(content) & >=50 & <=100
}

// 2 examples that use multi-byte, South Korean Hangul characters
multibyte_constrain_runes: "한글" // 2 runes composed of 6 bytes
// enforce that the field contains 5 or fewer runes
multibyte_constrain_runes: strings.MaxRunes(5)

multibyte_constrain_bytes: {
	// set up an alias to allow the following CUE to refer to it
	let content = "한글" // 2 runes composed of 6 bytes

	// this embedded scalar ultimately sets the value of the containing field,
	// "multibyte_constrain_bytes"
	content

	// enforce that "content" contains 5 or fewer bytes
	#bytes: len(content) & <=5
}

// 2 examples that use multi-byte, emoji characters
emoji_constrain_runes: "😄🥵🙃🥶" // 4 runes composed of 16 bytes
// enforce that the field contains 10 or more runes
emoji_constrain_runes: strings.MinRunes(10)

emoji_constrain_bytes: {
	// set up an alias to allow the following CUE to refer to it
	let content = "😄🥵🙃🥶" // 4 runes composed of 16 bytes

	// this embedded scalar ultimately sets the value of the containing field,
	// "emoji_constrain_bytes"
	content

	// enforce that "content" contains 10 or more bytes
	#bytes: len(content) & >=10
}
TERMINAL
$ cue vet .:example
ascii_constrain_runes: invalid value "only ASCII characters" (does not satisfy strings.MaxRunes(8)):
    ./example.cue:8:46
    ./example.cue:6:24
    ./example.cue:8:24
    ./example.cue:8:63
ascii_constrain_bytes.#bytes: invalid value 21 (out of bound >=50):
    ./example.cue:19:25
    ./example.cue:19:10
multibyte_constrain_bytes.#bytes: invalid value 6 (out of bound <=5):
    ./example.cue:36:25
    ./example.cue:36:10
emoji_constrain_runes: invalid value "😄🥵🙃🥶" (does not satisfy strings.MinRunes(10)):
    ./example.cue:42:24
    ./example.cue:40:24
    ./example.cue:42:41