$substr

The $substr operator returns a substring of a string, starting at a specified index and with a specified length.

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📌 Syntax

{ "$substr": [ <string>, <start>, <length> ] }

• <string>: The source string expression
• <start>: Zero-based start index
• <length>: Number of characters to return

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✅ Base Example 1 – Extract First 3 Letters

📥 Input Document

{ "product": "Shoes" }

📌 Expression

{ "$substr": ["$product", 0, 3] }

📤 Output

"Sho"

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✅ Base Example 2 – Get File Extension

📥 Input Document

{ "filename": "invoice2024.pdf" }

📌 Expression

{ "$substr": ["$filename", 12, 3] }

📤 Output

"pdf"

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🧱 Ecommerce Example – Shorten Brand Prefix from SKU

📌 Pipeline

[
  { "$unwind": "$items" },
  {
    "$project": {
      "shortSku": { "$substr": ["$items.sku", 0, 5] },
      "product": "$items.name"
    }
  }
]

📥 Input Document

{
  "items": [
    { "sku": "BRAND12345", "name": "Backpack" },
    { "sku": "DELTA98765", "name": "Laptop" }
  ]
}

📤 Output

[
  { "shortSku": "BRAND", "product": "Backpack" },
  { "shortSku": "DELTA", "product": "Laptop" }
]

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🔧 Common Use Cases

• Extract prefixes, suffixes, or sections
• Parse embedded strings
• Simplify field values for display

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🔗 Related Operators

• $substrBytes, $substrCP, $slice, $split, $indexOfBytes

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🧠 Notes

• $substr is deprecated in favor of $substrBytes and $substrCP for Unicode correctness.
• Start index out of range returns an empty string.

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