What do lazy and greedy mean in the context of regular expressions

Question

What are these two terms in an understandable way

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Taken From www regular-expressions info  Greediness  Greedy quantifiers first tries to repeat the token as many times as possible  and gradually gives up matches as the engine backtracks to find  an overall match   Laziness  Lazy quantifier first repeats the token as few times as required  and gradually expands the match as the engine backtracks through the regex to find an overall match

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Greedy means your expression will match as large a group as possible  lazy means it will match the smallest group possible   For this string   abcdefghijklmc   and this expression   a  c   A greedy match will match the whole string  and a lazy match will match just the first abc

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Best shown by example  String  192 168 1 1 and a greedy regex  b   b You might think this would give you the 1st octet but is actually matches against the whole string  Why  Because the   is greedy and a greedy match matches every character in 192 168 1 1 until it reaches the end of the string  This is the important bit  Now it starts to backtrack one character at a time until it finds a match for the 3rd token   b    If the string a 4GB text file and 192 168 1 1 was at the start you could easily see how this backtracking would cause an issue   To make a regex non greedy  lazy  put a question mark after your greedy search e g             What happens now is token 2      finds a match  regex moves along a character and then tries the next token   b  rather than token 2       So it creeps along gingerly

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Greedy will consume as much as possible  From http   www regular-expressions info repeat html we see the example of trying to match HTML tags with  lt    gt   Suppose you have the following    lt em gt Hello World lt  em gt    You may think that  lt    gt     means any non newline character and   means one or more  would only match the  lt em gt  and the  lt  em gt   when in reality it will be very greedy  and go from the first  lt  to the last  gt   This means it will match  lt em gt Hello World lt  em gt  instead of what you wanted   Making it lazy   lt     gt   will prevent this  By adding the   after the    we tell it to repeat as few times as possible  so the first  gt  it comes across  is where we want to stop the matching   I d encourage you to download RegExr  a great tool that will help you explore Regular Expressions - I use it all the time

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From Regular expression     The standard quantifiers in regular   expressions are greedy  meaning they   match as much as they can  only giving   back as necessary to match the   remainder of the regex        By using a lazy quantifier  the   expression tries the minimal match   first

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As far as I know  most regex engine is greedy by default  Add a question mark at the end of quantifier will enable lazy match   As  Andre S mentioned in comment    Greedy  Keep searching until condition is not satisfied  Lazy  Stop searching once condition is satisfied    Refer to the example below for what is greedy and what is lazy   import java util regex Matcher  import java util regex Pattern   public class Test       public static void main String args             String money    100000000999           String greedyRegex    100 0             Pattern pattern   Pattern compile greedyRegex           Matcher matcher   pattern matcher money           while matcher find                 System out println  I m greeedy and I want     matcher group       dollars  This is the most I can get                        String lazyRegex    100 0              pattern   Pattern compile lazyRegex           matcher   pattern matcher money           while matcher find                 System out println  I m too lazy to get so much money  only     matcher group       dollars is enough for me                         The result is   I m greeedy and I want 100000000 dollars  This is the most I can get   I m too lazy to get so much money  only 100 dollars is enough for me

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------------------- ----------------- ------------------------------    Greedy quantifier   Lazy quantifier          Description              ------------------- ----------------- ------------------------------                                          Star Quantifier  0 or more                                             Plus Quantifier  1 or more                                             Optional Quantifier  0 or 1       n                   n                Quantifier  exactly n             n                   n                Quantifier  n or more             n m                 n m              Quantifier  between n and m     ------------------- ----------------- ------------------------------       Add a   to a quantifier to make it ungreedy i e lazy     Example  test string   stackoverflow greedy reg expression   s  o output  stackoverflow lazy reg expression   s   o output  stackoverflow

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Greedy means it will consume your pattern until there are none of them left and it can look no further   Lazy will stop as soon as it will encounter the first pattern you requested   One common example that I often encounter is  s - s   of a regex   0-9  2  s - s   0-9  7    The first  s  is classified as greedy because of   and will look as many white spaces as possible after the digits are encountered and then look for a dash character  -   Where as the second  s   is lazy because of the present of    which means that it will look the first white space character and stop right there

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Greedy  means match longest possible string    Lazy  means match shortest possible string   For example  the greedy h  l matches  hell  in  hello  but the lazy h   l matches  hel

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Greedy Quantifiers are like the IRS ATO If it   s there  they   ll take it all  The IRS matches with this regex      50 000 This will match everything  See here for an example  Greedy-example Non-greedy quantifiers - they take as little as they can If I ask for a tax refund  the IRS sudden becomes non-greedy  and they use this quantifier     2 5     0-9    against this input   50 000 The first group is non-needy and only matches  5     so I get a  5 refund against the  50 000 input  They re non-greedy  They take as little as possible  See here  Non-greedy-example  Why bother  It becomes important if you are trying to match certain parts of an expression  Sometimes you don t want to match everything  Hopefully that analogy will help you remember

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try to understand the following behavior       var input    0014 2    Regex r1   new Regex    d   0 1   d     Regex r2   new Regex    d   0 1   d      Console WriteLine r1 Match input  Value       0014 2  Console WriteLine r2 Match input  Value       0014 2   input     0014 2    Console WriteLine r1 Match input  Value       0014 2  Console WriteLine r2 Match input  Value        0014   input      0014 2    Console WriteLine r1 Match input  Value       0014 2  Console WriteLine r2 Match input  Value

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Greedy matching  The default behavior of regular expressions is to be greedy  That means it tries to extract as much as possible until it conforms to a pattern even when a smaller part would have been syntactically sufficient   Example   import re text     lt body gt Regex Greedy Matching Example  lt  body gt   re findall   lt    gt    text    gt     lt body gt Regex Greedy Matching Example  lt  body gt      Instead of matching till the first occurrence of          it extracted the whole string  This is the default greedy or    take it all    behavior of regex   Lazy matching  on the other hand     takes as little as possible     This can be effected by adding a   at the end of the pattern   Example   re findall   lt     gt    text    gt     lt body gt      lt  body gt      If you want only the first match to be retrieved  use the search method instead   re search   lt     gt    text  group     gt    lt body gt     Source  Python Regex Examples

[regex] What do 'lazy' and 'greedy' mean in the context of regular expressions?

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