How to Parse CSV Files

Problem

Your CSV parser breaks on fields that contain commas, quotes, or newlines inside quoted values.

Prerequisites

• Understanding of CSV format
• Knowledge of RFC 4180 quoting rules

Steps

1. 1. Understand RFC 4180 rules

   Fields containing commas, double quotes, or newlines must be enclosed in double quotes. Quotes inside quoted fields are escaped by doubling them (""). A proper parser follows these rules rather than simply splitting on commas.

   Input

   "Smith, John",30

   Output

   one field: 'Smith, John'

2. 2. Use a standards-compliant parser

   Use a CSV parser that implements RFC 4180 rather than a naive string split. The parser should handle quoted fields, escaped quotes, and multiline values. Specify the delimiter, quote character, and encoding explicitly.

   Input

   CSV with quoted fields

   Output

   correctly parsed rows

3. 3. Handle edge cases

   Account for BOM at the start of the file, different line endings (CRLF vs LF), empty fields, and trailing delimiters. Decide on a policy for rows with inconsistent field counts. These edge cases are common in real-world CSV files.

   Input

   BOM + CRLF + empty fields

   Output

   handled gracefully

Expected result

Correctly parsed CSV data where quoted fields, embedded delimiters, and multiline values are handled properly.

Common errors

Naive split on comma breaks quoted fields

Never use a simple string.split(','). Use a proper CSV parser that respects quoting rules.

BOM appears as a hidden character in the first field

Detect and strip the UTF-8 BOM (EF BB BF) before parsing, or use a parser that handles it automatically.

Troubleshooting

• If rows are misaligned, check for unquoted fields containing commas or inconsistent line endings.
• For very large files, use a streaming CSV parser to process rows one at a time.

Security limits

• CSV injection: cells starting with =, +, -, or @ can execute formulas in spreadsheet apps. Sanitize if output will be opened in Excel.