V tej vadnici boste spoznali regularne izraze (RegEx) in uporabili Pythonov modul re za delo z RegEx (s pomočjo primerov).
Predmet gular Ex kompresije (regularnim izrazom) je zaporedje znakov, ki opredeljuje iskalni vzorec. Na primer
^a… s$
Zgornja koda definira vzorec RegEx. Vzorec je: poljuben pet črkovni niz, ki se začne z a in konča s .
Vzorec, definiran z uporabo RegEx, lahko uporabimo za ujemanje z nizom.
| Izraz | Vrvica | Ujemajo? |
|---|---|---|
^a… s$ | abs | Ni zadetka |
alias | Tekma | |
abyss | Tekma | |
Alias | Ni zadetka | |
An abacus | Ni zadetka |
Python ima modul, imenovan reza delo z RegEx. Tu je primer:
import re pattern = '^a… s$' test_string = 'abyss' result = re.match(pattern, test_string) if result: print("Search successful.") else: print("Search unsuccessful.")
Tu smo uporabili re.match()funkcijo za iskanje vzorca znotraj test_string. Če je iskanje uspešno, metoda vrne predmet ujemanja. Če ne, se vrne None.
V modulu re je določeno še več funkcij za delo z RegEx. Preden to preučimo, se naučimo samih regularnih izrazov.
Če že poznate osnove RegExa, pojdite na Python RegEx.
Določite vzorec z uporabo RegEx
Za določitev regularnih izrazov se uporabljajo metaznaki. V zgornjem primeru ^in $so metaznaki.
MetaCharacters
Metaznaki so znaki, ki jih stroj RegEx razlaga na poseben način. Tu je seznam metaznakov:
(). $ * +? () () |
() - oglati oklepaji
V oglatih oklepajih je določen nabor znakov, ki se jim želite ujemati.
| Izraz | Vrvica | Ujemajo? |
|---|---|---|
(abc) | a | 1 tekma |
ac | 2 tekmi | |
Hey Jude | Ni zadetka | |
abc de ca | 5 tekem |
Tu (abc)se bo ujemalo če niz se potrudiš, da se ujemajo vsebuje katerega koli od a, bali c.
S pomočjo -notranjih oglatih oklepajev lahko določite tudi obseg znakov .
(a-e)je enako kot(abcde).(1-4)je enako kot(1234).(0-39)je enako kot(01239).
Nabor znakov lahko dopolnite (obrnete) z uporabo ^znaka za oznako na začetku oglatega oklepaja.
(^abc)pomeni kateri koli znak, razen a ali b ali c.(^0-9)pomeni kateri koli nedimestni znak.
.- Obdobje
Točka se ujema s katerim koli znakom (razen z novo vrstico '').
| Izraz | Vrvica | Ujemajo? |
|---|---|---|
… | a | Ni zadetka |
ac | 1 tekma | |
acd | 1 tekma | |
acde | 2 ujemanja (vsebuje 4 znake) |
^- Caret
Simbol kare ^se uporablja za preverjanje, ali se niz začne z določenim znakom.
| Izraz | Vrvica | Ujemajo? |
|---|---|---|
^a | a | 1 tekma |
abc | 1 tekma | |
bac | Ni zadetka | |
^ab | abc | 1 tekma |
acb | Ni ujemanja (začne se z, avendar mu ne sledi b) |
$- Dolar
Simbol dolarja $se uporablja za preverjanje, ali se niz konča z določenim znakom.
| Izraz | Vrvica | Ujemajo? |
|---|---|---|
a$ | a | 1 tekma |
formula | 1 tekma | |
cab | Ni zadetka |
*- Zvezdica
Simbol zvezde se *ujema z nič ali več ponovitvami vzorca, ki mu ostane.
| Izraz | Vrvica | Ujemajo? |
|---|---|---|
ma*n | mn | 1 tekma |
man | 1 tekma | |
maaan | 1 tekma | |
main | Ni ujemanja ( ane sledi n) |
|
woman | 1 tekma |
+- Plus
Simbol plus se +ujema z enim ali več ponovitvami vzorca, ki mu ostane.
| Izraz | Vrvica | Ujemajo? |
|---|---|---|
ma+n | mn | Ni ujemanja (brez aznaka) |
man | 1 tekma | |
maaan | 1 tekma | |
main | Ni ujemanja (a ne sledi n) | |
woman | 1 tekma |
? - 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.
Luknja se uporablja za pobeg različnih znakov, vključno z vsemi metaznaki. Vendar pa z uporabo predpone r zdravljenje obravnavate kot običajen znak.
Primer 7: Surovi niz z uporabo predpone r
import re string = ' and are escape sequences.' result = re.findall(r'()', string) print(result) # Output: ('', '')
