Regex - take only the first occurrence of a word in Python?

Question

In a string, there are several occurrences of a word, but I want to take only the first occurrence of this, how to do?

Below comes 'first' twice, but I wish only the first.

re.findall(r'primeiro',' o primeiro o segundo primeiro novamente')

Thanks in advance.

Answer 1

Due to the lack of clarity of the question, it can be interpreted in the following ways:

• How to know if a word is present in a text?
• How to get index of the first occurrence of a word in a text?
• How to get the slice inside a string where the first occurrence of a word in a text is found?

Regardless of what the question actually is, no regex is used for any of the three cases. Regular expressions are an expensive and cumbersome processing feature and should only be used to find patterns of characters or bytes never to find a defined word because there are less costly features.

Don’t misunderstand, I love working with Regex but despite the nice name, regular expressions are a case of linear grammar within the Chomsky Hierarchy ie using regex you are loading a parser(deterministic finite automaton) into your program which means you are giving up memory resources and processing time. So use Regex yes, but when necessary for example:

• find words in a text containing special spelling or abject.
• find words in a text containing certain vowel encounters.
• special alphanumeric sequences.
• break text using variable separators.
• separate text into lexical symbols.
• anything that can be related to searching and searching based on character repetition patterns.

After the introduction we go to(s) question(s).

How to know if a word is present in a text?

To know if a word is present in a text use the python operator in. Operators in and not in salute. x in s returns True if x is contained in s and False otherwise. x not in s returns the denial of x in s.

s = ' o primeiro o segundo primeiro novamente'

print('primeiro' in s)                          #True
print('segundo' in s)                           #True
print('terceiro' in s)                          #False

How to get index of the first occurrence of a word in a text?

To get index of the first occurrence of a word in a text use the builtin method str.find().

str.find(sub[, start[, end]])
Returns the smallest index in the string where the substring sub is found inside the slice s[start:end]. Optional arguments such as start and end are interpreted as in slicing notation. Returns -1 if sub is not located.

s = ' o primeiro o segundo primeiro novamente'

print(s.find('primeiro'))                       #3
print(s.find('segundo'))                        #14
print(s.find('terceiro'))                       #-1

How to get the slice inside a string where the first occurrence of a word in a text is found?

To get slice inside a string where the first occurrence of a word in a text is found simply add index of the first occurrence of the word plus its length obtained with the builtin function Len(). Remembering that not always the length of a string is visual length of the same string

s = ' o primeiro o segundo primeiro novamente'

for p in ("primeiro", "segundo", "terceiro"):
  if p not in s:
    print(f"Palavra \"{p}\" não encontrada.")
  else:
    print((p, i:= s.find(p), len(p) + i))

#('primeiro', 3, 11)
#('segundo', 14, 21)
#Palavra "terceiro" não encontrada.

Answer 2

Use re.search(), works a little differently, follows the example:

import re

print(re.findall(r'primeiro',' o primeiro o segundo primeiro novamente'))
print("\n")
print(re.search(r'primeiro',' o primeiro o segundo primeiro novamente'))

Returns:

['primeiro', 'primeiro']

<re.Match object; span=(3, 11), match='primeiro'>

Answer 3

The methods re.findall and re.search will return occurrences in a list and a Match Object, respectively:

['primeiro', 'primeiro']
<re.Match object; span=(3, 11), match='primeiro'>

Match Object, result of the method re.search has a method span() that returns a tuple. Its items indicate the contents of the desired "substring" beginning and end within the initial string, respectively. You can access the contents of the desired string as in the example:

string = ' o primeiro o segundo primeiro novamente'
resultado = re.search(r'primeiro', string)
comeco, fim = resultado.span() # (3, 11)
print(string[comeco:fim]) # primeiro

If you prefer, the method Match.group can be used to return the first full occurrence of Match Object:

resultado.group() # primeiro