How to split but ignore separators in quoted strings, in python?

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Question :

How to split but ignore separators in quoted strings, in python?

I need to split a string like this, on semicolons. But I don’t want to split on semicolons that are inside of a string (‘ or “). I’m not parsing a file; just a simple string with no line breaks.

part 1;"this is ; part 2;";'this is ; part 3';part 4;this "is ; part" 5

Result should be:

  • part 1
  • “this is ; part 2;”
  • ‘this is ; part 3’
  • part 4
  • this “is ; part” 5

I suppose this can be done with a regex but if not; I’m open to another approach.

Asked By: Sylvain


Answer #1:

Most of the answers seem massively over complicated. You don’t need back references. You don’t need to depend on whether or not re.findall gives overlapping matches. Given that the input cannot be parsed with the csv module so a regular expression is pretty well the only way to go, all you need is to call re.split with a pattern that matches a field.

Note that it is much easier here to match a field than it is to match a separator:

import re
data = """part 1;"this is ; part 2;";'this is ; part 3';part 4;this "is ; part" 5"""
PATTERN = re.compile(r'''((?:[^;"']|"[^"]*"|'[^']*')+)''')
print PATTERN.split(data)[1::2]

and the output is:

['part 1', '"this is ; part 2;"', "'this is ; part 3'", 'part 4', 'this "is ; part" 5']

As Jean-Luc Nacif Coelho correctly points out this won’t handle empty groups correctly. Depending on the situation that may or may not matter. If it does matter it may be possible to handle it by, for example, replacing ';;' with ';<marker>;' where <marker> would have to be some string (without semicolons) that you know does not appear in the data before the split. Also you need to restore the data after:

>>> marker = ";!$%^&;"
>>> [r.replace(marker[1:-1],'') for r in PATTERN.split("aaa;;aaa;'b;;b'".replace(';;', marker))[1::2]]
['aaa', '', 'aaa', "'b;;b'"]

However this is a kludge. Any better suggestions?

Answered By: Sylvain

Answer #2:

re.split(''';(?=(?:[^'"]|'[^']*'|"[^"]*")*$)''', data)

Each time it finds a semicolon, the lookahead scans the entire remaining string, making sure there’s an even number of single-quotes and an even number of double-quotes. (Single-quotes inside double-quoted fields, or vice-versa, are ignored.) If the lookahead succeeds, the semicolon is a delimiter.

Unlike Duncan’s solution, which matches the fields rather than the delimiters, this one has no problem with empty fields. (Not even the last one: unlike many other split implementations, Python’s does not automatically discard trailing empty fields.)

Answered By: Duncan

Answer #3:

>>> a='A,"B,C",D'
>>> a.split(',')
['A', '"B', 'C"', 'D']

It failed. Now try csv module
>>> import csv
>>> from StringIO import StringIO
>>> data = StringIO(a)
>>> data
<StringIO.StringIO instance at 0x107eaa368>
>>> reader = csv.reader(data, delimiter=',') 
>>> for row in reader: print row
Answered By: Alan Moore

Answer #4:

Here is an annotated pyparsing approach:

from pyparsing import (printables, originalTextFor, OneOrMore, 
    quotedString, Word, delimitedList)

# unquoted words can contain anything but a semicolon
printables_less_semicolon = printables.replace(';','')

# capture content between ';'s, and preserve original text
content = originalTextFor(
    OneOrMore(quotedString | Word(printables_less_semicolon)))

# process the string
print delimitedList(content, ';').parseString(test)


['part 1', '"this is ; part 2;"', "'this is ; part 3'", 'part 4', 
 'this "is ; part" 5']

By using pyparsing’s provided quotedString, you also get support for escaped quotes.

You also were unclear how to handle leading whitespace before or after a semicolon delimiter, and none of your fields in your sample text has any. Pyparsing would parse “a; b ; c” as:

['a', 'b', 'c']

Answer #5:

You appears to have a semi-colon seperated string. Why not use the csv module to do all the hard work?

Off the top of my head, this should work

import csv 
from StringIO import StringIO 

line = '''part 1;"this is ; part 2;";'this is ; part 3';part 4;this "is ; part" 5'''

data = StringIO(line) 
reader = csv.reader(data, delimiter=';') 
for row in reader: 
    print row 

This should give you something like
("part 1", "this is ; part 2;", 'this is ; part 3', "part 4", "this "is
part"" 5"")

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