What is the difference between re.search and re.match?
Question:
What is the difference between the search() and match() functions in the Python re module?
I’ve read the Python 2 documentation (Python 3 documentation), but I never seem to remember it. I keep having to look it up and re-learn it. I’m hoping that someone will answer it clearly with examples so that (perhaps) it will stick in my head. Or at least I’ll have a better place to return with my question and it will take less time to re-learn it.
Answers:
re.match is anchored at the beginning of the string. That has nothing to do with newlines, so it is not the same as using ^ in the pattern.
As the re.match documentation says:
If zero or more characters at the
beginning of string match the regular expression pattern, return a
corresponding MatchObject instance.
Return None if the string does not
match the pattern; note that this is
different from a zero-length match.
Note: If you want to locate a match
anywhere in string, use search()
instead.
re.search searches the entire string, as the documentation says:
Scan through string looking for a
location where the regular expression
pattern produces a match, and return a
corresponding MatchObject instance.
Return None if no position in the
string matches the pattern; note that
this is different from finding a
zero-length match at some point in the
string.
So if you need to match at the beginning of the string, or to match the entire string use match. It is faster. Otherwise use search.
The documentation has a specific section for match vs. search that also covers multiline strings:
Python offers two different primitive
operations based on regular
expressions: match checks for a match
only at the beginning of the string,
while search checks for a match
anywhere in the string (this is what
Perl does by default).
Note that match may differ from search
even when using a regular expression
beginning with '^': '^' matches only
at the start of the string, or in
MULTILINE mode also immediately
following a newline. The “match”
operation succeeds only if the pattern
matches at the start of the string
regardless of mode, or at the starting
position given by the optional pos
argument regardless of whether a
newline precedes it.
Now, enough talk. Time to see some example code:
# example code:
string_with_newlines = """something
someotherthing"""
import re
print re.match('some', string_with_newlines) # matches
print re.match('someother',
string_with_newlines) # won't match
print re.match('^someother', string_with_newlines,
re.MULTILINE) # also won't match
print re.search('someother',
string_with_newlines) # finds something
print re.search('^someother', string_with_newlines,
re.MULTILINE) # also finds something
m = re.compile('thing$', re.MULTILINE)
print m.match(string_with_newlines) # no match
print m.match(string_with_newlines, pos=4) # matches
print m.search(string_with_newlines,
re.MULTILINE) # also matches
re.match attempts to match a pattern at the beginning of the string. re.search attempts to match the pattern throughout the string until it finds a match.
re.search searches for the pattern throughout the string, whereas re.match does not search the pattern; if it does not, it has no other choice than to match it at start of the string.
search ⇒ find something anywhere in the string and return a match object.
match ⇒ find something at the beginning of the string and return a match object.
You can refer the below example to understand the working of re.match and re.search
a = "123abc"
t = re.match("[a-z]+",a)
t = re.search("[a-z]+",a)
re.match will return none, but re.search will return abc.
The difference is, re.match() misleads anyone accustomed to Perl, grep, or sed regular expression matching, and re.search() does not. 🙂
More soberly, As John D. Cook remarks, re.match() “behaves as if every pattern has ^ prepended.” In other words, re.match('pattern') equals re.search('^pattern'). So it anchors a pattern’s left side. But it also doesn’t anchor a pattern’s right side: that still requires a terminating $.
Frankly given the above, I think re.match() should be deprecated. I would be interested to know reasons it should be retained.
match is much faster than search, so instead of doing regex.search("word") you can do regex.match((.*?)word(.*?)) and gain tons of performance if you are working with millions of samples.
This comment from @ivan_bilan under the accepted answer above got me thinking if such hack is actually speeding anything up, so let’s find out how many tons of performance you will really gain.
I prepared the following test suite:
import random
import re
import string
import time
LENGTH = 10
LIST_SIZE = 1000000
def generate_word():
word = [random.choice(string.ascii_lowercase) for _ in range(LENGTH)]
word = ''.join(word)
return word
wordlist = [generate_word() for _ in range(LIST_SIZE)]
start = time.time()
[re.search('python', word) for word in wordlist]
print('search:', time.time() - start)
start = time.time()
[re.match('(.*?)python(.*?)', word) for word in wordlist]
print('match:', time.time() - start)
I made 10 measurements (1M, 2M, …, 10M words) which gave me the following plot:
As you can see, searching for the pattern 'python' is faster than matching the pattern '(.*?)python(.*?)'.
Python is smart. Avoid trying to be smarter.
Much shorter:
-
search scans through the whole string.
-
match scans only the beginning of the string.
Following Ex says it:
>>> a = "123abc"
>>> re.match("[a-z]+",a)
None
>>> re.search("[a-z]+",a)
abc
Quick answer
re.search('test', ' test') # returns a Truthy match object (because the search starts from any index)
re.match('test', ' test') # returns None (because the search start from 0 index)
re.match('test', 'test') # returns a Truthy match object (match at 0 index)
What is the difference between the search() and match() functions in the Python re module?
I’ve read the Python 2 documentation (Python 3 documentation), but I never seem to remember it. I keep having to look it up and re-learn it. I’m hoping that someone will answer it clearly with examples so that (perhaps) it will stick in my head. Or at least I’ll have a better place to return with my question and it will take less time to re-learn it.
re.match is anchored at the beginning of the string. That has nothing to do with newlines, so it is not the same as using ^ in the pattern.
As the re.match documentation says:
If zero or more characters at the
beginning of string match the regular expression pattern, return a
correspondingMatchObjectinstance.
ReturnNoneif the string does not
match the pattern; note that this is
different from a zero-length match.Note: If you want to locate a match
anywhere in string, usesearch()
instead.
re.search searches the entire string, as the documentation says:
Scan through string looking for a
location where the regular expression
pattern produces a match, and return a
correspondingMatchObjectinstance.
ReturnNoneif no position in the
string matches the pattern; note that
this is different from finding a
zero-length match at some point in the
string.
So if you need to match at the beginning of the string, or to match the entire string use match. It is faster. Otherwise use search.
The documentation has a specific section for match vs. search that also covers multiline strings:
Python offers two different primitive
operations based on regular
expressions:matchchecks for a match
only at the beginning of the string,
whilesearchchecks for a match
anywhere in the string (this is what
Perl does by default).Note that
matchmay differ fromsearch
even when using a regular expression
beginning with'^':'^'matches only
at the start of the string, or in
MULTILINEmode also immediately
following a newline. The “match”
operation succeeds only if the pattern
matches at the start of the string
regardless of mode, or at the starting
position given by the optionalpos
argument regardless of whether a
newline precedes it.
Now, enough talk. Time to see some example code:
# example code:
string_with_newlines = """something
someotherthing"""
import re
print re.match('some', string_with_newlines) # matches
print re.match('someother',
string_with_newlines) # won't match
print re.match('^someother', string_with_newlines,
re.MULTILINE) # also won't match
print re.search('someother',
string_with_newlines) # finds something
print re.search('^someother', string_with_newlines,
re.MULTILINE) # also finds something
m = re.compile('thing$', re.MULTILINE)
print m.match(string_with_newlines) # no match
print m.match(string_with_newlines, pos=4) # matches
print m.search(string_with_newlines,
re.MULTILINE) # also matches
re.match attempts to match a pattern at the beginning of the string. re.search attempts to match the pattern throughout the string until it finds a match.
re.search searches for the pattern throughout the string, whereas re.match does not search the pattern; if it does not, it has no other choice than to match it at start of the string.
search ⇒ find something anywhere in the string and return a match object.
match ⇒ find something at the beginning of the string and return a match object.
You can refer the below example to understand the working of re.match and re.search
a = "123abc"
t = re.match("[a-z]+",a)
t = re.search("[a-z]+",a)
re.match will return none, but re.search will return abc.
The difference is, re.match() misleads anyone accustomed to Perl, grep, or sed regular expression matching, and re.search() does not. 🙂
More soberly, As John D. Cook remarks, re.match() “behaves as if every pattern has ^ prepended.” In other words, re.match('pattern') equals re.search('^pattern'). So it anchors a pattern’s left side. But it also doesn’t anchor a pattern’s right side: that still requires a terminating $.
Frankly given the above, I think re.match() should be deprecated. I would be interested to know reasons it should be retained.
match is much faster than search, so instead of doing regex.search("word") you can do regex.match((.*?)word(.*?)) and gain tons of performance if you are working with millions of samples.
This comment from @ivan_bilan under the accepted answer above got me thinking if such hack is actually speeding anything up, so let’s find out how many tons of performance you will really gain.
I prepared the following test suite:
import random
import re
import string
import time
LENGTH = 10
LIST_SIZE = 1000000
def generate_word():
word = [random.choice(string.ascii_lowercase) for _ in range(LENGTH)]
word = ''.join(word)
return word
wordlist = [generate_word() for _ in range(LIST_SIZE)]
start = time.time()
[re.search('python', word) for word in wordlist]
print('search:', time.time() - start)
start = time.time()
[re.match('(.*?)python(.*?)', word) for word in wordlist]
print('match:', time.time() - start)
I made 10 measurements (1M, 2M, …, 10M words) which gave me the following plot:
As you can see, searching for the pattern 'python' is faster than matching the pattern '(.*?)python(.*?)'.
Python is smart. Avoid trying to be smarter.
Much shorter:
-
searchscans through the whole string. -
matchscans only the beginning of the string.
Following Ex says it:
>>> a = "123abc"
>>> re.match("[a-z]+",a)
None
>>> re.search("[a-z]+",a)
abc
Quick answer
re.search('test', ' test') # returns a Truthy match object (because the search starts from any index)
re.match('test', ' test') # returns None (because the search start from 0 index)
re.match('test', 'test') # returns a Truthy match object (match at 0 index)