5 Ways to Find the Index of a Substring in Python

During file processing in Python, you’ll face many situations in which you’ll need to search the index for certain substrings. In this article, I have covered many methods to find the index of the substring in Python.  

1. str.find()

str.find() will return the lowest index of the substring mentioned. start and end parameters are optional. If start and end parameters are mentioned, it will search the substring within the start and endindex. It will return -1 if the substring is not found.

str.find(sub,start,end)

Example 1: Using str.find() Method

The string is banana.

The substring is an.

The substring occurs two times in the string.

str.find(“an”) returns the lowest index of the substring an.

s1="banana"
print (s1.find("an"))
#Output:1

Example 2. Using str.find() Method With the Start Parameter 

The substring is an.

The start parameter is 2. It will start searching the substring an from index two.

s1="banana"
print (s1.find("an",2))
#Output:3

Example 3: Substring Not found

The substring is ba.

The start parameter is 1 and the stop parameter is 5. It will start searching the substring from index 1 to index 5 (excluded).

Since the substring is not found in the string within the given index, it returns -1.

s1="banana"
print (s1.find("ba",1,5))
#Output:-1

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2. str.rfind()

str.rfind() will return the highest index of the substring mentioned. start and end parameters are optional. If start and end parameters are mentioned, it will search the substring within the start and end index. It will return -1 if the substring isn’t found.

str.rfind(sub,start,end)

Example 1: Using str.rfind() Method

The string is banana.

The substring is an.

The substring occurs two times in the string.

str.find(“an”) returns the highest index of the substring an.

s1="banana"
print (s1.rfind("an"))
#Output:3

Example 2: Using str.rfind() Method With Start and End Parameters 

The substring is an.

The start and end parameters are 1 and 4, respectively. It will start searching the substring from index 1 and index 4 (excluded).

s1="banana"
print (s1.rfind("an",1,4))
#Output:1

Example 3: Substring Is Not Found

The substring is no.

Since the substring is not found in the string, it returns -1.

s1="banana"
print (s1.rfind("no"))
#Output:-1

3. str.index()

str.index() will return the lowest index of the substring mentioned. It will raise a ValueError if the substring is not found.

Example 1: Using str.index() Method

s1="banana"
print (s1.index("an"))
#Output:1

Example 2: Using str.index() Method With Start and End Parameters 

s1="banana"
print (s1.index("an",2,6))
#Output:3

Example 3: Substring Is Not Found

s1="banana"
print (s1.index("no"))
#Output:ValueError: substring not found

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4. str.rindex()

str.rindex() will return the highest index of the substring mentioned. It will raise a ValueError if the substring is not found.

Example 1: Using str.rindex() Method

s1="banana"
print (s1.rindex("an"))
#Output:3

Example 2: Using str.index() Method With Start and End Parameters 

s1="banana"
print (s1.rindex("an",0,4))
#Output:1

Example 3: Substring Is Not Found

s1="banana"
print (s1.rindex("no"))
#Output:ValueError: substring not found

5. re.search()

re.search(pattern, string, flags=0)

• re.search (pattern, string): We have to mention the pattern to be searched in the string.
• The return type matches the object that contains the starting and ending index of that pattern (substring).
• We can find the start and end indices from the match object using match.start() and match.end().

Match.start([group])
Match.end([group])

• We can get the start and end indices in tuple format using match.span(

Match.span([group])

Example 1. Using re.search()

import re
string = 'banana'
pattern = 'an'
match=(re.search(pattern, string))

#Returns match object
print (match)#Output:<re.Match object; span=(1, 3), match='an'>

#getting the starting index using match.start()
print (match.start()) #Output: 1

#Getting the start and end index in tuple format using match.span()
print (match.span()) #Output: (1,3)

Example 2: Substring Is Not Found

import re
string = 'banana'
pattern = 'no'
match=(re.search(pattern, string))

#Returns match object
print (match)#Output: None

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What to Know About Each Python String Method

Below is a summary of what you need to remember about each Python string method. 

• str.find(), str,index(): These return the lowest index of the substring.
• str.rfind(), str.rindex(): These return the highest index of the substring.
• re.search(): This returns the match object that contains the starting and ending indices of the substring.
• str.find(), str.rfind(): These methods return -1 when a substring is not found.
• str.index(),str.rindex(): These raise a ValueError when a substring is not found.
• re.search(): This returns None when a substring is not found.