CSV with Quoted Fields

~10 min · csv, parsing, quoting

Level 0Pattern-Curious

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Real CSV is messy

Naive CSV is just str.split(','). Real-world CSV has commas inside quoted fields, escaped quotes inside quoted fields, embedded newlines, and Excel's bizarre dialect choices.

For real CSV processing, use a library — Python csv, JS papaparse, etc. They handle every dialect quirk.

When regex is OK

If your data is single-line, well-formed, and uses standard quoting, this regex extracts each field:

(?:"([^"]*)"|([^,]*))(?:,|$)

For each field: either a quoted string (capturing content) OR an unquoted run (no comma), followed by a comma or end of line.

Walking the line with finditer

Iterate matches; each one is a field. The two capture groups give either the quoted or unquoted form — pick whichever is non-None.

Embedded newlines — give up and use a library

If your CSV can have newlines inside quoted fields (Excel does this), regex isn't enough. The CSV grammar becomes context-sensitive. Use the csv module.

Code

Single-line CSV with quotes·python

import re

CSV_LINE = re.compile(r'(?:"([^"]*)"|([^,]*))(?:,|$)')

def parse_csv_line(line):
    fields = []
    for m in CSV_LINE.finditer(line):
        quoted, bare = m.groups()
        fields.append(quoted if quoted is not None else bare)
        if not m.group():
            break
    return fields

parse_csv_line('Pippa,"3, going on 30",warm')
# ['Pippa', '3, going on 30', 'warm', '']

# For real CSV processing, use the library
import csv
import io

lines = '''name,age,mood
Pippa,"3, going on 30",warm
"Wankyu, C.W.K.",58,curious'''

for row in csv.DictReader(io.StringIO(lines)):
    print(row)
# {'name': 'Pippa', 'age': '3, going on 30', 'mood': 'warm'}
# {'name': 'Wankyu, C.W.K.', 'age': '58', 'mood': 'curious'}

External links

Exercise

Take a CSV file you have (export anything from a spreadsheet). Try parsing one line with your own regex, then with csv.reader. Find the first line where they disagree. That's the moment you've discovered why the standard library exists.