How to extract a substring from inside a string in Python?

Let's say I have a string 'gfgfdAAA1234ZZZuijjk' and I want to extract just the '1234' part.

'gfgfdAAA1234ZZZuijjk'

'1234'

I only know what will be the few characters directly before AAA, and after ZZZ the part I am interested in 1234.

AAA

ZZZ

1234

With sed it is possible to do something like this with a string:

sed

echo "$STRING" | sed -e "s|.*AAA(.*)ZZZ.*|1|"


And this will give me 1234 as a result.

1234

How to do the same thing in Python?

12 Answers
12

Using regular expressions - documentation for further reference

import re

text = 'gfgfdAAA1234ZZZuijjk'

m = re.search('AAA(.+?)ZZZ', text)
if m:
found = m.group(1)

# found: 1234


or:

import re

text = 'gfgfdAAA1234ZZZuijjk'

try:
found = re.search('AAA(.+?)ZZZ', text).group(1)
except AttributeError:
# AAA, ZZZ not found in the original string
found = '' # apply your error handling

# found: 1234


>>> s = 'gfgfdAAA1234ZZZuijjk'
>>> start = s.find('AAA') + 3
>>> end = s.find('ZZZ', start)
>>> s[start:end]
'1234'


Then you can use regexps with the re module as well, if you want, but that's not necessary in your case.

re

s

s.find

-1

s[begin:end]

import re

re.search(r"(?<=AAA).*?(?=ZZZ)", your_text).group(0)


The above as-is will fail with an AttributeError if there are no "AAA" and "ZZZ" in your_text

AttributeError

your_text

your_text.partition("AAA")[2].partition("ZZZ")[0]


The above will return an empty string if either "AAA" or "ZZZ" don't exist in your_text.

your_text

PS Python Challenge?

import re
print re.search('AAA(.*?)ZZZ', 'gfgfdAAA1234ZZZuijjk').group(1)


AttributeError: 'NoneType' object has no attribute 'groups'

You can use re module for that:

>>> import re
>>> re.compile(".*AAA(.*)ZZZ.*").match("gfgfdAAA1234ZZZuijjk").groups()
('1234,)


With sed it is possible to do something like this with a string:

echo "$STRING" | sed -e "s|.*AAA(.*)ZZZ.*|1|"

echo "$STRING" | sed -e "s|.*AAA(.*)ZZZ.*|1|"

And this will give me 1234 as a result.

You could do the same with re.sub function using the same regex.

re.sub

>>> re.sub(r'.*AAA(.*)ZZZ.*', r'1', 'gfgfdAAA1234ZZZuijjk')
'1234'


In basic sed, capturing group are represented by (..), but in python it was represented by (..).

(..)

(..)

You can find first substring with this function in your code (by character index). Also, you can find what is after a substring.

def FindSubString(strText, strSubString, Offset=None):
try:
Start = strText.find(strSubString)
if Start == -1:
return -1 # Not Found
else:
if Offset == None:
Result = strText[Start+len(strSubString):]
elif Offset == 0:
return Start
else:
AfterSubString = Start+len(strSubString)
Result = strText[AfterSubString:AfterSubString + int(Offset)]
return Result
except:
return -1

# Example:

Text = "Thanks for contributing an answer to Stack Overflow!"
subText = "to"

print("Start of first substring in a text:")
start = FindSubString(Text, subText, 0)
print(start); print("")

print("Exact substring in a text:")
print(Text[start:start+len(subText)]); print("")

print("What is after substring "%s"?" %(subText))
print(FindSubString(Text, subText))

# Your answer:

Text = "gfgfdAAA1234ZZZuijjk"
subText1 = "AAA"
subText2 = "ZZZ"

AfterText1 = FindSubString(Text, subText1, 0) + len(subText1)
BeforText2 = FindSubString(Text, subText2, 0)

print("nYour answer:n%s" %(Text[AfterText1:BeforText2]))


Just in case somebody will have to do the same thing that I did. I had to extract everything inside parenthesis in a line. For example, if I have a line like 'US president (Barack Obama) met with ...' and I want to get only 'Barack Obama' this is solution:

regex = '.*((.*?)).*'
matches = re.search(regex, line)
line = matches.group(1) + 'n'


I.e. you need to block parenthesis with slash sign. Though it is a problem about more regular expressions that Python.

slash

Also, in some cases you may see 'r' symbols before regex definition. If there is no r prefix, you need to use escape characters like in C. Here is more discussion on that.

>>> s = '/tmp/10508.constantstring'
>>> s.split('/tmp/')[1].split('constantstring')[0].strip('.')


you can do using just one line of code

>>> import re

>>> re.findall(r'd{1,5}','gfgfdAAA1234ZZZuijjk')

>>> ['1234']


result will receive list...

In python, extracting substring form string can be done using findall method in regular expression (re) module.

findall

re

>>> import re
>>> s = 'gfgfdAAA1234ZZZuijjk'
>>> ss = re.findall('AAA(.+)ZZZ', s)
>>> print ss
['1234']


One liners that return other string if there was no match.
Edit: improved version uses next function, replace "not-found" with something else if needed:

next

"not-found"

import re
res = next( (m.group(1) for m in [re.search("AAA(.*?)ZZZ", "gfgfdAAA1234ZZZuijjk" ),] if m), "not-found" )


My other method to do this, less optimal, uses regex 2nd time, still didn't found a shorter way:

import re
res = ( ( re.search("AAA(.*?)ZZZ", "gfgfdAAA1234ZZZuijjk") or re.search("()","") ).group(1) )


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