String Validation

JSON Schema provides several keywords for validating string instances.

minLength

Specifies the minimum length of a string.

Syntax

The value must be a non-negative integer.

{
  "minLength": 5
}

Validation Rules

A string instance is valid if its length is greater than or equal to the specified value. The length is defined as the number of Unicode code points in the string.

Examples

{
  "type": "string",
  "minLength": 3
}

Valid instances:

"abc"
"hello"
"12345"

Invalid instances:

"ab"
"a"
""

Default Behavior

Omitting minLength is equivalent to minLength: 0, which means empty strings are allowed.

maxLength

Specifies the maximum length of a string.

Syntax

The value must be a non-negative integer.

{
  "maxLength": 10
}

Validation Rules

A string instance is valid if its length is less than or equal to the specified value. The length is defined as the number of Unicode code points in the string.

Examples

{
  "type": "string",
  "maxLength": 5
}

Valid instances:

"abc"
"hello"
""

Invalid instances:

"toolong"
"abcdef"

pattern

Validates that a string matches a regular expression.

Syntax

The value must be a string containing a valid regular expression.

{
  "pattern": "^[A-Z][a-z]+$"
}

Validation Rules

A string instance is valid if the regular expression matches the string successfully. Important: Regular expressions are not implicitly anchored. To match the entire string, use anchors like ^ (start) and $ (end).

Regular Expression Dialect

The pattern should be a valid regular expression according to ECMA-262. See the interoperability considerations in the specification for details.

Examples

Email Pattern

{
  "type": "string",
  "pattern": "^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$"
}

Valid instances:

"user@example.com"
"john.doe@company.co.uk"

Invalid instances:

"invalid.email"
"@example.com"

Phone Number Pattern

{
  "type": "string",
  "pattern": "^\\+?[1-9]\\d{1,14}$"
}

Valid instances:

"+14155552671"
"14155552671"

Alphanumeric Pattern

{
  "type": "string",
  "pattern": "^[a-zA-Z0-9]+$"
}

Valid instances:

"abc123"
"TEST"
"123"

Invalid instances:

"hello world"
"test@123"

Partial Matching

{
  "pattern": "JSON"
}

This matches any string containing “JSON” anywhere:

"I love JSON"
"JSON Schema"
"myJSON"

Combining String Keywords

Multiple string validation keywords can be used together.

Username Validation

{
  "type": "string",
  "minLength": 3,
  "maxLength": 20,
  "pattern": "^[a-zA-Z0-9_-]+$"
}

This validates a username that:

Is between 3 and 20 characters long
Contains only letters, numbers, underscores, and hyphens

Valid instances:

"john_doe"
"user123"
"my-username"

Invalid instances:

"ab"                  // Too short
"verylongusernamehere123" // Too long
"user@name"          // Invalid character

Password Validation

{
  "type": "string",
  "minLength": 8,
  "pattern": "^(?=.*[a-z])(?=.*[A-Z])(?=.*\\d)(?=.*[@$!%*?&])[A-Za-z\\d@$!%*?&]+$"
}

This validates a password that:

Is at least 8 characters long
Contains at least one lowercase letter, one uppercase letter, one digit, and one special character

Product Code

{
  "type": "string",
  "minLength": 8,
  "maxLength": 8,
  "pattern": "^[A-Z]{3}-\\d{4}$"
}

Valid instance:

"ABC-1234"

Unicode Considerations

The minLength and maxLength keywords count Unicode code points, not bytes or visible characters.

{
  "minLength": 3
}

The string "😀👍✨" has a length of 3 code points and is valid, even though it may appear as multiple characters depending on how emojis are rendered.

Core Keywords

Applicator Keywords

Validation Keywords

Meta-Data

minLength

Syntax

Validation Rules

Examples

Default Behavior

maxLength

Syntax

Validation Rules

Examples

pattern

Syntax

Validation Rules

Regular Expression Dialect

Examples

Email Pattern

Phone Number Pattern

Alphanumeric Pattern

Partial Matching

Combining String Keywords

Username Validation

Password Validation

Product Code

Unicode Considerations

​minLength

​Syntax

​Validation Rules

​Examples

​Default Behavior

​maxLength

​Syntax

​Validation Rules

​Examples

​pattern

​Syntax

​Validation Rules

​Regular Expression Dialect

​Examples

​Email Pattern

​Phone Number Pattern

​Alphanumeric Pattern

​Partial Matching

​Combining String Keywords

​Username Validation

​Password Validation

​Product Code

​Unicode Considerations

minLength

Syntax

Validation Rules

Examples

Default Behavior

maxLength

Syntax

Validation Rules

Examples

pattern

Syntax

Validation Rules

Regular Expression Dialect

Examples

Email Pattern

Phone Number Pattern

Alphanumeric Pattern

Partial Matching

Combining String Keywords

Username Validation

Password Validation

Product Code

Unicode Considerations