Difference Between Re.Search and Re.Match

What is the difference between re.search and re.match?

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

Python: Difference between re.match(pattern) v/s re.search('^' + pattern)

You should take a look at Python's re.search() vs. re.match() document which clearly mentions about the other difference which is:

Note however that in MULTILINE mode match() only matches at the beginning of the string, whereas using search() with a regular expression beginning with '^' will match at the beginning of each line.

>>> re.match('X', 'A\nB\nX', re.MULTILINE) # No match

>>> re.search('^X', 'A\nB\nX', re.MULTILINE) # Match

<_sre.SRE_Match object; span=(4, 5), match='X'>

The first difference (for future readers) being:

Python offers two different primitive operations based on regular expressions: re.match() checks for a match only at the beginning of the string, while re.search() checks for a match anywhere in the string (this is what Perl does by default).

For example:

>>> re.match("c", "abcdef") # No match

>>> re.search("c", "abcdef") # Match

<_sre.SRE_Match object; span=(2, 3), match='c'>

Regular expressions beginning with '^' can be used with search() to restrict the match at the beginning of the string:

>>> re.match("c", "abcdef") # No match

>>> re.search("^c", "abcdef") # No match

>>> re.search("^a", "abcdef") # Match

<_sre.SRE_Match object; span=(0, 1), match='a'>

Differences between re.match, re.search, re.fullmatch

Giving credit for @Ruzihm's answer since parts of my answer derive from his.

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Quick overview

A quick rundown of the differences:

• re.match is anchored at the start ^pattern
  
  
  • Ensures the string begins with the pattern
• re.fullmatch is anchored at the start and end of the pattern ^pattern$
  
  
  • Ensures the full string matches the pattern (can be especially useful with alternations as described here)
• re.search is not anchored pattern
  
  
  • Ensures the string contains the pattern

A more in-depth comparison of re.match vs re.search can be found here

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With examples:

aa            # string
a|aa          # regex

re.match:     a
re.search:    a
re.fullmatch: aa

 

ab            # string
^a            # regex

re.match:     a
re.search:    a
re.fullmatch: # None (no match)

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So what about \A and \Z anchors?

The documentation states the following:

Python offers two different primitive operations based on regular
expressions: re.match() checks for a match only at the beginning of
the string, while re.search() checks for a match anywhere in the
string (this is what Perl does by default).

And in the Pattern.fullmatch section it says:

If the whole string matches this regular expression, return a corresponding match object.

And, as initially found and quoted by Ruzihm in his answer:

Note however that in MULTILINE mode match() only matches at the
beginning of the string, whereas using search() with a regular
expression beginning with ^ will match at the beginning of each
line.

>>> re.match('X', 'A\nB\nX', re.MULTILINE)  # No match
>>> re.search('^X', 'A\nB\nX', re.MULTILINE)  # Match
<re.Match object; span=(4, 5), match='X'>

\A^A
B
X$\Z

# re.match('X', s)                  no match
# re.search('^X', s)                no match

# ------------------------------------------
# and the string above when re.MULTILINE is enabled effectively becomes

\A^A$
^B$
^C$\Z

# re.match('X', s, re.MULTILINE)    no match
# re.search('^X', s, re.MULTILINE)  match X

With regards to \A and \Z, neither performs differently for re.MULTILINE since \A and \Z are effectively the only ^ and $ in the whole string.

So using \A and \Z with any of the three methods yields the same results.

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Answer (line anchors vs string anchors)

What this tells me is that re.match and re.fullmatch don't match line anchors ^ and $ respectively, but that they instead match string anchors \A and \Z respectively.

Trying to understand re.search() vs re.findall()

The method re.findall returns a list of matched substrings, but the method re.search returns a match object, you can recover the full matched substring like this.

b.group() # 'Eventin queue contains 5 elements, first element is 20 minutes old'

What you were seeing, <_sre.SRE_Match object; span=(0, 66), match='Eventin queue contains 5 elements, first element >, is only a representation of the object.

same result from both re.search() and re.match() are the same, but not the same by comparison operator

Because the Match type does not have a custom __eq__ method, the equality operation will always return False, unless it's the exact same Match instance.

The default behavior for equality comparison (== and !=) is based on
the identity of the objects. Hence, equality comparison of instances
with the same identity results in equality, and equality comparison of
instances with different identities results in inequality.

https://docs.python.org/3/reference/expressions.html#value-comparisons

Every time you call re.match or re.search, the return value will be a different Match object, even when the input data is exactly the same.

>>> needle, haystack = 's', 'spam'                                                                                                                                                                                                  
>>> re.match(needle, haystack) == re.match(needle, haystack)                                                                                                                                                                        
    False

Why use re.match(), when re.search() can do the same thing?

"Why" questions are hard to answer. As a matter of fact, you could define the function re.match() like this:

def match(pattern, string, flags):
    return re.search(r"\A(?:" + pattern + ")", string, flags)

(because \A always matches at the start of the string, regardless of the re.M flag status´).

So re.match is a useful shortcut but not strictly necessary. It's especially confusing for Java programmers who have Pattern.matches() which anchors the search to the start and end of the string (which is probably a more common use case than just anchoring to the start).

It's different for the match and search methods of regex objects, though, as Eric has pointed out.

Python regex - understanding the difference between match and search

When calling the function re.match specifically, the ^ character does have little meaning because this function begins the matching process at the beginning of the line. However, it does have meaning for other functions in the re module, and when calling match on a compiled regular expression object.

For example:

text = """\
Mares eat oats
and does eat oats
"""

print re.findall('^(\w+)', text, re.MULTILINE) 

This prints:

['Mares', 'and']

With a re.findall() and re.MULTILINE enabled, it gives you the first word (with no leading whitespace) on each line of your text.

It might be useful if doing something more complex, like lexical analysis with regular expressions, and passing into the compiled regular expression a starting position in the text it should start matching at (which you can choose to be the ending position from the previous match). See the documentation for RegexObject.match method.

Simple lexer / scanner as an example:

text = """\
Mares eat oats
and does eat oats
"""

pattern = r"""
(?P<firstword>^\w+)
|(?P<lastword>\w+$)
|(?P<word>\w+)
|(?P<whitespace>\s+)
|(?P<other>.)
"""

rx = re.compile(pattern, re.MULTILINE | re.VERBOSE)

def scan(text):
    pos = 0
    m = rx.match(text, pos)
    while m:
        toktype = m.lastgroup
        tokvalue = m.group(toktype)
        pos = m.end()
        yield toktype, tokvalue
        m = rx.match(text, pos)

for tok in scan(text):
    print tok

which prints

('firstword', 'Mares')
('whitespace', ' ')
('word', 'eat')
('whitespace', ' ')
('lastword', 'oats')
('whitespace', '\n')
('firstword', 'and')
('whitespace', ' ')
('word', 'does')
('whitespace', ' ')
('word', 'eat')
('whitespace', ' ')
('lastword', 'oats')
('whitespace', '\n')

This distinguishes between types of word; a word at the beginning of a line, a word at the end of a line, and any other word.

re.match versus re.findall

re.match matches the pattern from the start of the string. re.findall however searches for occurrences of the pattern anywhere in the string.

If you have the pattern "mail failure" and the string:

subject = "=?UTF-8?B?0JLQsNGI0LUg0YHQvtC+0LHRidC10L3QuNC1INC90LUg0LTQvtGB0YLQsNCy0LvQtdC90L4=?=. Mail failure."

re.match will return None because the string does not start with "mail failure". re.findall though will return a match because the string contains "mail failure".

re.match vs re.search

In your match, the first .* is greedy, it is matching as much as it can, including numbers.
If you make it less greedy, it will work:

 .*?([0-9]{1,})Y.*

(PS I think this greedy issue doesn't make it a fair comparison of re.search and re.match)

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