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

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

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

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

And this will give me 1234 as a result.

How to do the same thing in Python?

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.

regular expression

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

string methods

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.

PS Python Challenge?

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

Surprised that nobody has mentioned this which is my quick version for one-off scripts:

>>> x = 'gfgfdAAA1234ZZZuijjk'
>>> x.split('AAA')[1].split('ZZZ')[0]
'1234'

you can do using just one line of code

>>> import re

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

>>> ['1234']

result will receive list…

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|"

And this will give me 1234 as a result.

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

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

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

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

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

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]))

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

text = 'I want to find a string between two substrings'
left = 'find a '
right = 'between two'

print(text[text.index(left)+len(left):text.index(right)])

Gives

string

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.

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.

Using PyParsing

import pyparsing as pp

word = pp.Word(pp.alphanums)

s = 'gfgfdAAA1234ZZZuijjk'
rule = pp.nestedExpr('AAA', 'ZZZ')
for match in rule.searchString(s):
    print(match)

which yields:

[['1234']]

Here’s a solution without regex that also accounts for scenarios where the first substring contains the second substring. This function will only find a substring if the second marker is after the first marker.

def find_substring(string, start, end):
    len_until_end_of_first_match = string.find(start) + len(start)
    after_start = string[len_until_end_of_first_match:]
    return string[string.find(start) + len(start):len_until_end_of_first_match + after_start.find(end)]

Another way of doing it is using lists (supposing the substring you are looking for is made of numbers, only) :

string = 'gfgfdAAA1234ZZZuijjk'
numbersList = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9']
output = []

for char in string:
    if char in numbersList: output.append(char)

print(f"output: {''.join(output)}")
### output: 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:

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) )