I den här handledningen lär du dig om reguljära uttryck (RegEx) och använder Pythons re-modul för att arbeta med RegEx (med hjälp av exempel).
En Re gular Ex pression (RegEx) är en sekvens av tecken som definierar ett sökmönster. Till exempel,
^a… s$
Ovanstående kod definierar ett RegEx-mönster. Mönstret är: valfri fembokstavssträng som börjar med a och slutar med s .
Ett mönster definierat med hjälp av RegEx kan användas för att matcha mot en sträng.
| Uttryck | Sträng | Matchad? |
|---|---|---|
^a… s$ | abs | Ingen match |
alias | Match | |
abyss | Match | |
Alias | Ingen match | |
An abacus | Ingen match |
Python har en modul som heter för reatt arbeta med RegEx. Här är ett exempel:
import re pattern = '^a… s$' test_string = 'abyss' result = re.match(pattern, test_string) if result: print("Search successful.") else: print("Search unsuccessful.")
Här använde vi re.match()funktionen för att söka mönster i teststrängen. Metoden returnerar ett matchningsobjekt om sökningen lyckas. Om inte, återkommer den None.
Det finns andra flera funktioner som definieras i re-modulen för att arbeta med RegEx. Innan vi utforskar det, låt oss lära oss om reguljära uttryck själva.
Om du redan känner till grunderna i RegEx, hoppa till Python RegEx.
Ange mönster med hjälp av RegEx
För att ange reguljära uttryck används metatecken. I exemplet ovan ^och $är metatecken.
Metakaraktärer
Metatecken är karaktärer som tolkas på ett speciellt sätt av en RegEx-motor. Här är en lista över metatecken:
(). $ * +? () () |
() - Hakparentes
Hakparenteser anger en uppsättning tecken som du vill matcha.
| Uttryck | Sträng | Matchad? |
|---|---|---|
(abc) | a | 1 match |
ac | 2 matcher | |
Hey Jude | Ingen match | |
abc de ca | 5 matcher |
Här (abc)kommer att matcha om strängen du försöker matcha innehåller någon av a, beller c.
Du kan också ange ett teckenintervall med -hakparenteser.
(a-e)är samma som(abcde).(1-4)är samma som(1234).(0-39)är samma som(01239).
Du kan komplettera (invertera) teckenuppsättningen med hjälp av markeringssymbolen ^i början av en hakparentes.
(^abc)betyder alla tecken utom a eller b eller c.(^0-9)betyder alla icke-siffriga tecken.
.- Period
En period matchar varje enskilt tecken (utom ny linje '').
| Uttryck | Sträng | Matchad? |
|---|---|---|
… | a | Ingen match |
ac | 1 match | |
acd | 1 match | |
acde | 2 träffar (innehåller 4 tecken) |
^- Vaktmästare
Caret-symbolen ^används för att kontrollera om en sträng börjar med en viss karaktär.
| Uttryck | Sträng | Matchad? |
|---|---|---|
^a | a | 1 match |
abc | 1 match | |
bac | Ingen match | |
^ab | abc | 1 match |
acb | Ingen matchning (börjar med amen inte följt av b) |
$- Dollar
Dollarsymbolen $används för att kontrollera om en sträng slutar med ett visst tecken.
| Uttryck | Sträng | Matchad? |
|---|---|---|
a$ | a | 1 match |
formula | 1 match | |
cab | Ingen match |
*- Stjärna
Stjärnsymbolen *matchar noll eller fler förekomster av det mönster som är kvar.
| Uttryck | Sträng | Matchad? |
|---|---|---|
ma*n | mn | 1 match |
man | 1 match | |
maaan | 1 match | |
main | Ingen matchning ( aföljs inte av n) |
|
woman | 1 match |
+- Plus
Plussymbolen +matchar en eller flera förekomster av det mönster som är kvar.
| Uttryck | Sträng | Matchad? |
|---|---|---|
ma+n | mn | Ingen matchning (inget atecken) |
man | 1 match | |
maaan | 1 match | |
main | Ingen matchning (a följs inte av n) | |
woman | 1 match |
? - 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.
Backlash används för att undkomma olika karaktärer inklusive alla metatecken. Att använda r-prefixet gör behandla som en normal karaktär.
Exempel 7: Rå sträng med r-prefix
import re string = ' and are escape sequences.' result = re.findall(r'()', string) print(result) # Output: ('', '')
