U ovom vodiču naučit ćete o regularnim izrazima (RegEx) i koristiti Pythonov modul re za rad s RegExom (uz pomoć primjera).
Re Gular Ex protuzračne (regularni izraz) je niz znakova koji definira uzorak za pretraživanje. Na primjer,
^a… s$
Gornji kod definira RegEx obrazac. Uzorak je: bilo koji niz od pet slova koji počinje s a i završava s .
Uzorak definiran pomoću RegEx-a može se koristiti za podudaranje sa nizom.
| Izraz | Niz | Podudarno? |
|---|---|---|
^a… s$ | abs | Nema podudaranja |
alias | Podudarnost | |
abyss | Podudarnost | |
Alias | Nema podudaranja | |
An abacus | Nema podudaranja |
Python ima modul imenovan reza rad s RegExom. Evo primjera:
import re pattern = '^a… s$' test_string = 'abyss' result = re.match(pattern, test_string) if result: print("Search successful.") else: print("Search unsuccessful.")
Ovdje smo koristili re.match()funkciju za pretraživanje uzorka unutar test_stringa. Metoda vraća objekt podudaranja ako je pretraživanje uspješno. Ako nije, vraća se None.
Postoji još nekoliko funkcija definiranih u modulu re za rad s RegExom. Prije nego što to istražimo, naučimo same regularne izraze.
Ako već znate osnove RegExa, prijeđite na Python RegEx.
Navedite obrazac pomoću RegEx-a
Za specificiranje regularnih izraza koriste se metaznakovi. U gornjem primjeru su ^i $metaznakovi.
MetaCharacters
Metaznakovi su znakovi koje RegExov mehanizam tumači na poseban način. Evo popisa metaznaka:
(). $ * +? () () |
() - Uglate zagrade
Uglatim zagradama naveden je skup znakova kojima se želite podudarati.
| Izraz | Niz | Podudarno? |
|---|---|---|
(abc) | a | 1 utakmica |
ac | 2 utakmice | |
Hey Jude | Nema podudaranja | |
abc de ca | 5 utakmica |
Ovdje (abc)će odgovarati ako niz pokušavate uskladiti sadrži bilo koji od a, bili c.
Također možete odrediti raspon znakova pomoću -unutarstih zagrada.
(a-e)je isto što i(abcde).(1-4)je isto što i(1234).(0-39)je isto što i(01239).
Možete nadopuniti (invertirati) skup znakova pomoću ^simbola znaka na početku uglate zagrade.
(^abc)označava bilo koji znak osim a ili b ili c.(^0-9)znači bilo koji ne-znamenkasti znak.
.- Razdoblje
Točka se podudara s bilo kojim pojedinačnim znakom (osim novog retka '').
| Izraz | Niz | Podudarno? |
|---|---|---|
… | a | Nema podudaranja |
ac | 1 utakmica | |
acd | 1 utakmica | |
acde | 2 podudaranja (sadrži 4 znaka) |
^- Caret
Simbol karete ^koristi se za provjeru započinje li niz određenim znakom.
| Izraz | Niz | Podudarno? |
|---|---|---|
^a | a | 1 utakmica |
abc | 1 utakmica | |
bac | Nema podudaranja | |
^ab | abc | 1 utakmica |
acb | Nema podudaranja (započinje, aali ne slijedi b) |
$- Dolar
Simbol dolara $koristi se za provjeru završava li niz određenim znakom.
| Izraz | Niz | Podudarno? |
|---|---|---|
a$ | a | 1 utakmica |
formula | 1 utakmica | |
cab | Nema podudaranja |
*- Zvijezda
Simbol zvijezde *podudara se s nula ili više pojavljivanja uzorka koji mu je ostavljen.
| Izraz | Niz | Podudarno? |
|---|---|---|
ma*n | mn | 1 utakmica |
man | 1 utakmica | |
maaan | 1 utakmica | |
main | Nema podudaranja ( ane slijedi ga n) |
|
woman | 1 utakmica |
+- Plus
Simbol plus +odgovara jednoj ili više pojava uzorka koji mu je ostavljen.
| Izraz | Niz | Podudarno? |
|---|---|---|
ma+n | mn | Nema podudaranja (nema aznaka) |
man | 1 utakmica | |
maaan | 1 utakmica | |
main | Nema podudaranja (a ne slijedi n) | |
woman | 1 utakmica |
? - Question Mark
The question mark symbol ? matches zero or one occurrence of the pattern left to it.
| Expression | String | Matched? |
|---|---|---|
ma?n | mn | 1 match |
man | 1 match | |
maaan | No match (more than one a character) |
|
main | No match (a is not followed by n) | |
woman | 1 match |
() - Braces
Consider this code: (n,m). This means at least n, and at most m repetitions of the pattern left to it.
| Expression | String | Matched? |
|---|---|---|
a(2,3) | abc dat | No match |
abc daat | 1 match (at daat) |
|
aabc daaat | 2 matches (at aabc and daaat) |
|
aabc daaaat | 2 matches (at aabc and daaaat) |
Let's try one more example. This RegEx (0-9)(2, 4) matches at least 2 digits but not more than 4 digits
| Expression | String | Matched? |
|---|---|---|
(0-9)(2,4) | ab123csde | 1 match (match at ab123csde) |
12 and 345673 | 3 matches (12, 3456, 73) |
|
1 and 2 | No match |
| - Alternation
Vertical bar | is used for alternation (or operator).
| Expression | String | Matched? |
|---|---|---|
a|b | cde | No match |
ade | 1 match (match at ade) |
|
acdbea | 3 matches (at acdbea) |
Here, a|b match any string that contains either a or b
() - Group
Parentheses () is used to group sub-patterns. For example, (a|b|c)xz match any string that matches either a or b or c followed by xz
| Expression | String | Matched? |
|---|---|---|
(a|b|c)xz | ab xz | No match |
abxz | 1 match (match at abxz) |
|
axz cabxz | 2 matches (at axzbc cabxz) |
- Backslash
Backlash is used to escape various characters including all metacharacters. For example,
$a match if a string contains $ followed by a. Here, $ is not interpreted by a RegEx engine in a special way.
If you are unsure if a character has special meaning or not, you can put in front of it. This makes sure the character is not treated in a special way.
Special Sequences
Special sequences make commonly used patterns easier to write. Here's a list of special sequences:
A - Matches if the specified characters are at the start of a string.
| Expression | String | Matched? |
|---|---|---|
Athe | the sun | Match |
In the sun | No match |
- Matches if the specified characters are at the beginning or end of a word.
| Expression | String | Matched? |
|---|---|---|
foo | football | Match |
a football | Match | |
afootball | No match | |
foo | the foo | Match |
the afoo test | Match | |
the afootest | No match |
B - Opposite of . Matches if the specified characters are not at the beginning or end of a word.
| Expression | String | Matched? |
|---|---|---|
Bfoo | football | No match |
a football | No match | |
afootball | Match | |
fooB | the foo | No match |
the afoo test | No match | |
the afootest | Match |
d - Matches any decimal digit. Equivalent to (0-9)
| Expression | String | Matched? |
|---|---|---|
d | 12abc3 | 3 matches (at 12abc3) |
Python | No match |
D - Matches any non-decimal digit. Equivalent to (^0-9)
| Expression | String | Matched? |
|---|---|---|
D | 1ab34"50 | 3 matches (at 1ab34"50) |
1345 | No match |
s - Matches where a string contains any whitespace character. Equivalent to ( fv).
| Expression | String | Matched? |
|---|---|---|
s | Python RegEx | 1 match |
PythonRegEx | No match |
S - Matches where a string contains any non-whitespace character. Equivalent to (fv).
| Expression | String | Matched? |
|---|---|---|
S | a b | 2 matches (at a b) |
| No match |
w - Matches any alphanumeric character (digits and alphabets). Equivalent to (a-zA-Z0-9_). By the way, underscore _ is also considered an alphanumeric character.
| Expression | String | Matched? |
|---|---|---|
w | 12&": ;c | 3 matches (at 12&": ;c) |
%"> ! | No match |
W - Matches any non-alphanumeric character. Equivalent to (^a-zA-Z0-9_)
| Expression | String | Matched? |
|---|---|---|
W | 1a2%c | 1 match (at 1a2%c) |
Python | No match |
- Matches if the specified characters are at the end of a string.
| Expression | String | Matched? |
|---|---|---|
Python | I like Python | 1 match |
I like Python Programming | No match | |
Python is fun. | No match |
Tip: To build and test regular expressions, you can use RegEx tester tools such as regex101. This tool not only helps you in creating regular expressions, but it also helps you learn it.
Now you understand the basics of RegEx, let's discuss how to use RegEx in your Python code.
Python RegEx
Python has a module named re to work with regular expressions. To use it, we need to import the module.
import re
The module defines several functions and constants to work with RegEx.
re.findall()
The re.findall() method returns a list of strings containing all matches.
Example 1: re.findall()
# Program to extract numbers from a string import re string = 'hello 12 hi 89. Howdy 34' pattern = 'd+' result = re.findall(pattern, string) print(result) # Output: ('12', '89', '34')
If the pattern is not found, re.findall() returns an empty list.
re.split()
The re.split method splits the string where there is a match and returns a list of strings where the splits have occurred.
Example 2: re.split()
import re string = 'Twelve:12 Eighty nine:89.' pattern = 'd+' result = re.split(pattern, string) print(result) # Output: ('Twelve:', ' Eighty nine:', '.')
If the pattern is not found, re.split() returns a list containing the original string.
You can pass maxsplit argument to the re.split() method. It's the maximum number of splits that will occur.
import re string = 'Twelve:12 Eighty nine:89 Nine:9.' pattern = 'd+' # maxsplit = 1 # split only at the first occurrence result = re.split(pattern, string, 1) print(result) # Output: ('Twelve:', ' Eighty nine:89 Nine:9.')
By the way, the default value of maxsplit is 0; meaning all possible splits.
re.sub()
The syntax of re.sub() is:
re.sub(pattern, replace, string)
The method returns a string where matched occurrences are replaced with the content of replace variable.
Example 3: re.sub()
# Program to remove all whitespaces import re # multiline string string = 'abc 12 de 23 f45 6' # matches all whitespace characters pattern = 's+' # empty string replace = '' new_string = re.sub(pattern, replace, string) print(new_string) # Output: abc12de23f456
If the pattern is not found, re.sub() returns the original string.
You can pass count as a fourth parameter to the re.sub() method. If omited, it results to 0. This will replace all occurrences.
import re # multiline string string = 'abc 12 de 23 f45 6' # matches all whitespace characters pattern = 's+' replace = '' new_string = re.sub(r's+', replace, string, 1) print(new_string) # Output: # abc12de 23 # f45 6
re.subn()
The re.subn() is similar to re.sub() expect it returns a tuple of 2 items containing the new string and the number of substitutions made.
Example 4: re.subn()
# Program to remove all whitespaces import re # multiline string string = 'abc 12 de 23 f45 6' # matches all whitespace characters pattern = 's+' # empty string replace = '' new_string = re.subn(pattern, replace, string) print(new_string) # Output: ('abc12de23f456', 4)
re.search()
The re.search() method takes two arguments: a pattern and a string. The method looks for the first location where the RegEx pattern produces a match with the string.
If the search is successful, re.search() returns a match object; if not, it returns None.
match = re.search(pattern, str)
Example 5: re.search()
import re string = "Python is fun" # check if 'Python' is at the beginning match = re.search('APython', string) if match: print("pattern found inside the string") else: print("pattern not found") # Output: pattern found inside the string
Here, match contains a match object.
Match object
You can get methods and attributes of a match object using dir() function.
Some of the commonly used methods and attributes of match objects are:
match.group()
The group() method returns the part of the string where there is a match.
Example 6: Match object
import re string = '39801 356, 2102 1111' # Three digit number followed by space followed by two digit number pattern = '(d(3)) (d(2))' # match variable contains a Match object. match = re.search(pattern, string) if match: print(match.group()) else: print("pattern not found") # Output: 801 35
Here, match variable contains a match object.
Our pattern (d(3)) (d(2)) has two subgroups (d(3)) and (d(2)). You can get the part of the string of these parenthesized subgroups. Here's how:
>>> match.group(1) '801' >>> match.group(2) '35' >>> match.group(1, 2) ('801', '35') >>> match.groups() ('801', '35')
match.start(), match.end() and match.span()
The start() function returns the index of the start of the matched substring. Similarly, end() returns the end index of the matched substring.
>>> match.start() 2 >>> match.end() 8
The span() function returns a tuple containing start and end index of the matched part.
>>> match.span() (2, 8)
match.re and match.string
The re attribute of a matched object returns a regular expression object. Similarly, string attribute returns the passed string.
>>> match.re re.compile('(\d(3)) (\d(2))') >>> match.string '39801 356, 2102 1111'
We have covered all commonly used methods defined in the re module. If you want to learn more, visit Python 3 re module.
Using r prefix before RegEx
When r or R prefix is used before a regular expression, it means raw string. For example, '' is a new line whereas r'' means two characters: a backslash followed by n.
Lučenje se koristi za izbjegavanje različitih znakova, uključujući sve metaznake. Međutim, upotreba prefiksa r čini liječenje normalnim znakom.
Primjer 7: Sirovi niz pomoću prefiksa r
import re string = ' and are escape sequences.' result = re.findall(r'()', string) print(result) # Output: ('', '')
