Hows performs the regex: /.+? /?

Question

How would ". +?" regular expression work? Is the part. + Coinciding with anything written, huh? part says it can be or not? So, for example, this regular expression will match:

'cat'
'' (i.e. nothing is written, just an empty string)

Answer 1

In addition to what Hans Kesting has already said, the lazy multiplier will make an exact oposite from normal greedy factors: the possible match is kept as little as possible, and the rest of the regular expression is checked.

So, if you have a string aabaand check for a regular expression on it a.*b, the internal processing steps will be as follows:

• ain corresponds a .*b a aba
• .*in matches as well as greedy a .* ba a ba.*
  3. .*then matches a ab a
  4. .*then matches a aba

bin fails because there is no letter a.* b

• backtracking goes one step back, and .*now it will only match bbina ab a

bin still not workinga.* baab a

• backtracking goes one step back, and .*now only matches bina a ba

bin now matches in and have been met. a.* bbaa b a

So a complete match . aab a

(a.*?b), oposite, :

• a a .*?b a aba
• .* a .* ?b (*= ), .* (.*?),
• b a.*? b a a ba
  • backtracking .*
• .* a a ba
• b a.*? b aa b a .

, , aab a.

Answer 2

"+?" "+", "?" . "?" "+", "" " " , , .

, "a +?" regex "a" "caaat".

Answer 3

+? ( )

. , , .

/".+?"/ "def" ( "ghi" ) abc "def" "ghi" jkl, /".+"/ "def" "ghi" .

Answer 4

, Perl perldoc perlre.

"", .. ( ), . , , "?". , , "":

    *?     Match 0 or more times, not greedily
    +?     Match 1 or more times, not greedily
    ??     Match 0 or 1 time, not greedily
    {n}?   Match exactly n times, not greedily
    {n,}?  Match at least n times, not greedily
    {n,m}? Match at least n but not more than m times, not greedily

, , Perl . . , Perl "" .

    *+     Match 0 or more times and give nothing back
    ++     Match 1 or more times and give nothing back
    ?+     Match 0 or 1 time and give nothing back
    {n}+   Match exactly n times and give nothing back (redundant)
    {n,}+  Match at least n times and give nothing back
    {n,m}+ Match at least n but not more than m times and give nothing back

,

   'aaaa' =~ /a++a/

, a++ a . , perl , . , " " :

   /"(?:[^"\\]++|\\.)*+"/

, , . . (?>...); - . , :

   /"(?>(?:(?>[^"\\]+)|\\.)*)"/

Answer 5

inevitably, the regular expression will search for at least one character. I came across a case where an empty line did not pass this test already, it would be better to use it .*?or (.*)?instead, sometimes you need to specify a part of the line, which can be zero in curly brackets before the question mark, this helps. For example. \d{6}?will lead to an incorrect result, whereas if I said (\d{6})?in a line, say, for example:

preg_match("/shu\.(\d{6})?/", "shu.321456")

this will give true, and so will be a string "shu."without any int after period