python regex to return empty string

1
votes

I want to extract part of a string in a list which does not have a space followed by number in python.

# INPUT
text = ['bits', 'scrap 1.2', 'bits and pieces', 'junk 3.4.2']
# EXPECTED OUTPUT
output = ['bits', 'scrap', 'bits and pieces', 'junk']

I managed to do this using re.sub or re.split:

output = [re.sub(" [0-9].*", "", t) for t in text]
# OR
output = [re.split(' \d',t)[0] for t in text]

When I tried to use re.search and re.findall, it return me empty list or empty result.

[re.search('(.*) \d', t) for t in text]
#[None, <_sre.SRE_Match object; span=(0, 7), match='scrap 1'>, None, <_sre.SRE_Match object; span=(0, 6), match='junk 3'>]

[re.findall('(.*?) \d', t) for t in text]
#[[], ['scrap'], [], ['junk']]

Can anyone help me with the regex that can return expected output for re.search and re.findall?

1
pythonregex

1 Answers

4
votes

You may remove the digit-and-dot substrings at the end of the string only with

import re
text = ['bits', 'scrap 1.2', 'bits and pieces', 'junk 3.4.2']
print([re.sub(r'\s+\d+(?:\.\d+)*$', '', x) for x in text])
# => output = ['bits', 'scrap', 'bits and pieces', 'junk']

See the Python demo

The pattern is

  • \s+ - 1+ whitespaces (note: if those digits can be "glued" to some other text, replace + (one or more occurrences) with * quantifier (zero or more occurrences))
  • \d+ - 1 or more digits
  • (?:\.\d+)* - 0 or more sequences of
    • \. - a dot
    • \d+ - 1 or more digits
  • $ - end of string.

See the regex demo.

To do the same with re.findall, you can use 

# To get 'abc 5.6 def' (not 'abc') from 'abc 5.6 def 6.8.9'
re.findall(r'^(.*?)(?: \d[\d.]*)?$', x) # 
# To get 'abc' (not 'abc 5.6 def') from 'abc 5.6 def 6.8.9'
re.findall(r'^(.*?)(?: \d.*)?$', x) #

See this regex demo.

However, this regex is not efficient enough due to the .*? construct. Here,

  • ^ - start of string
  • (.*?) - Group 1: any zero or more chars other than line break chars (use re.DOTALL to match all) as few as possible (so that the next optional group could be tested at each position)
  • (?: \d[\d.]*)? -an optional non-capturing group matching
    • - a space
    • \d - a digit
    • [\d.]* - zero or more digits or . chars
    • (OR) .* - any 0+ chars other than line break chars, as many as possible
  • $ - end of string.