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 documentation  current 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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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     The resulting lines are surprisingly  actually not that surprisingly  straight  And the search function is  slightly  faster given this specific pattern combination  The moral of this test  Avoid overoptimizing your code

User · Answer

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

User · Answer

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     The resulting lines are surprisingly  actually not that surprisingly  straight  And the search function is  slightly  faster given this specific pattern combination  The moral of this test  Avoid overoptimizing your code

User · Answer

Much shorter    search scans through the whole string  match scans only the beginning of the string    Following Ex says it    gt  gt  gt  a    123abc   gt  gt  gt  re match   a-z    a  None  gt  gt  gt  re search   a-z    a  abc

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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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

search  rArr  find something anywhere in the string and return a match object   match  rArr  find something at the beginning of the string and return a match object

User · Answer

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

User · Answer

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

User · Answer

Much shorter    search scans through the whole string  match scans only the beginning of the string    Following Ex says it    gt  gt  gt  a    123abc   gt  gt  gt  re match   a-z    a  None  gt  gt  gt  re search   a-z    a  abc

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

search  rArr  find something anywhere in the string and return a match object   match  rArr  find something at the beginning of the string and return a match object

User · Answer

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

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