I denne opplæringen lærer du om regulære uttrykk (RegEx), og bruker Pythons re-modul til å jobbe med RegEx (ved hjelp av eksempler).
En Re gular Ex pression (regulært uttrykk) er en sekvens av tegn som definerer et søkemønster. For eksempel,
^a… s$
Ovennevnte kode definerer et RegEx-mønster. Mønsteret er: hvilken som helst fem bokstavstreng som begynner med a og slutter med s .
Et mønster definert ved hjelp av RegEx kan brukes til å matche mot en streng.
| Uttrykk | String | Matchet? |
|---|---|---|
^a… s$ | abs | Ingen treff |
alias | Kamp | |
abyss | Kamp | |
Alias | Ingen treff | |
An abacus | Ingen treff |
Python har en modul kalt for reå jobbe med RegEx. Her er et eksempel:
import re pattern = '^a… s$' test_string = 'abyss' result = re.match(pattern, test_string) if result: print("Search successful.") else: print("Search unsuccessful.")
Her brukte vi re.match()funksjon for å søke mønster i teststrengen. Metoden returnerer et matchobjekt hvis søket er vellykket. Hvis ikke, kommer den tilbake None.
Det er andre funksjoner som er definert i re-modulen for å jobbe med RegEx. Før vi utforsker det, la oss lære om regulære uttrykk selv.
Hvis du allerede vet det grunnleggende om RegEx, kan du hoppe til Python RegEx.
Spesifiser mønster ved hjelp av RegEx
For å spesifisere regulære uttrykk brukes metategn. I eksemplet ovenfor, ^og $er metategn.
Metakarakterer
Metategn er tegn som tolkes på en spesiell måte av en RegEx-motor. Her er en liste over metategn:
(). $ * +? () () |
() - Firkantede parenteser
Firkantede parenteser angir et sett med tegn du vil matche.
| Uttrykk | String | Matchet? |
|---|---|---|
(abc) | a | 1 kamp |
ac | 2 kamper | |
Hey Jude | Ingen treff | |
abc de ca | 5 kamper |
Her (abc)vil matche hvis strengen du prøver å matche inneholder noen av a, beller c.
Du kan også spesifisere en rekke tegn ved hjelp av -firkantede parenteser.
(a-e)er det samme som(abcde).(1-4)er det samme som(1234).(0-39)er det samme som(01239).
Du kan utfylle (invertere) tegnsettet ved å bruke inntegningssymbolet ^i begynnelsen av en firkantet parentes.
(^abc)betyr hvilket som helst tegn unntatt a eller b eller c.(^0-9)betyr et hvilket som helst ikke-sifret tegn.
.- Periode
En periode samsvarer med et enkelt tegn (unntatt ny linje '').
| Uttrykk | String | Matchet? |
|---|---|---|
… | a | Ingen treff |
ac | 1 kamp | |
acd | 1 kamp | |
acde | 2 treff (inneholder 4 tegn) |
^- Vaktmester
Salsymbolet ^brukes til å sjekke om en streng starter med et bestemt tegn.
| Uttrykk | String | Matchet? |
|---|---|---|
^a | a | 1 kamp |
abc | 1 kamp | |
bac | Ingen treff | |
^ab | abc | 1 kamp |
acb | Ingen kamp (starter med amen ikke etterfulgt av b) |
$- Dollar
Dollarsymbolet $brukes til å sjekke om en streng slutter med et bestemt tegn.
| Uttrykk | String | Matchet? |
|---|---|---|
a$ | a | 1 kamp |
formula | 1 kamp | |
cab | Ingen treff |
*- Stjerne
Stjernesymbolet *samsvarer med null eller flere forekomster av mønsteret som er igjen.
| Uttrykk | String | Matchet? |
|---|---|---|
ma*n | mn | 1 kamp |
man | 1 kamp | |
maaan | 1 kamp | |
main | Ingen kamp ( afølges ikke av n) |
|
woman | 1 kamp |
+- Pluss
Plussymbolet +samsvarer med en eller flere forekomster av mønsteret som er igjen.
| Uttrykk | String | Matchet? |
|---|---|---|
ma+n | mn | Ingen treff (ingen akarakter) |
man | 1 kamp | |
maaan | 1 kamp | |
main | Ingen treff (a følges ikke av n) | |
woman | 1 kamp |
? - 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 brukes til å unnslippe forskjellige tegn, inkludert alle metategn. Imidlertid gjør bruk av r prefiks behandle som en normal karakter.
Eksempel 7: Rå streng med r-prefiks
import re string = ' and are escape sequences.' result = re.findall(r'()', string) print(result) # Output: ('', '')
