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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: 
"[email protected]"
"[email protected]"
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.

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