Regex expressions in Java s vs s

Question

What s the difference between the following two expressions   x   x replaceAll    s        x   x replaceAll    s

User · Answer

The first one matches a single whitespace  whereas the second one matches one or many whitespaces  They re the so-called regular expression quantifiers  and they perform matches like this  taken from the documentation    Greedy quantifiers X   X  once or not at all X   X  zero or more times X   X  one or more times X n     X  exactly n times X n     X  at least n times X n m   X  at least n but not more than m times  Reluctant quantifiers X   X  once or not at all X   X  zero or more times X   X  one or more times X n     X  exactly n times X n     X  at least n times X n m   X  at least n but not more than m times  Possessive quantifiers X   X  once or not at all X   X  zero or more times X   X  one or more times X n     X  exactly n times X n     X  at least n times X n m   X  at least n but not more than m times

User · Answer

First of all you need to understand that final output of both the statements will be same i e  to remove all the spaces from given string   However x replaceAll    s         will be more efficient way of trimming spaces  if string can have multiple contiguous spaces  because of potentially less no of replacements due the to fact that regex   s  matches 1 or more spaces at once and replaces them with empty string   So even though you get the same output from both it is better to use   x replaceAll    s

User · Answer

Those two replaceAll calls will always produce the same result  regardless of what x is  However  it is important to note that the two regular expressions are not the same      s - matches single whitespace character     s  - matches sequence of one or more whitespace characters    In this case  it makes no difference  since you are replacing everything with an empty string  although it would be better to use   s  from an efficiency point of view   If you were replacing with a non-empty string  the two would behave differently

User · Answer

The first regex will match one whitespace character  The second regex will reluctantly match one or more whitespace characters  For most purposes  these two regexes are very similar  except in the second case  the regex can match more of the string  if it prevents the regex match from failing    from http   www coderanch com t 570917 java java regex-difference

[java] Regex expressions in Java, \\s vs. \\s+

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