Difficult PHP Nested Regular Expression

Question

Given the following string:

{start} SubPattern1 {end} 
....
{start} SubPattern2 {end}
....
{start} {start}SubPattern3{end} {end}

I need to find the regular expression that gives me the following result:

preg_match_all($regex, $string, $result);

print_r($result);

array(2
    0 => array(3
        0 => {start} SubPattern1 {end}
        1 => {start} SubPattern2 {end}
        2 => {start} {start}SubPattern3{end} {end}
    )
    1 => array(3
        0 => SubPattern1 
        1 => SubPattern2 
        2 => {start}SubPattern3{end} 
    )
)

Thank you!

Edit

For visual purpose I wrote a multiline string. But I need that the expression works even if all the text is in a single line string. eg:

{start}SubPattern1{end}{start}SubPattern2{end}{start}{start}SubPattern3{end}{end}

I don't understand why this question has been marked as duplicated. — Jorge Luis, Dec 16 '18 at 23:16

score 0 · Answer 1 · answered Dec 16 '18 at 15:27

0

You can use this

^{start}(.*){end}$

^ - Start of string.
{start} - Matches {start}.
(.*) - Matches anything except new line. (capturing group).
{end} - Matches {end}.
$ - End of the string.

Demo

answered Dec 16 '18 at 15:27

Code Maniac

33,907
4
28
50

Thanks! But that only works if the patterns are in different lines. I need that the expression works even if the string is in a single line. I'll do the aclaration in the original post. – Jorge Luis Dec 16 '18 at 15:53
@JorgeLuis look at it https://regex101.com/r/8ULlDM/3 – Code Maniac Dec 16 '18 at 15:55
that's so close, but not working in the last nested pattern. Thanks again. – Jorge Luis Dec 16 '18 at 16:07

The fourth bird · Accepted Answer · 2018-12-17T08:24:09.030

0

You could match {start} and {end} and use a capturing group and use recursion for the pattern:

{start}((?:(?:(?!{(?:start|end)}).)+|(?R))*){end}

For example:

$string = <<<DATA
{start} SubPattern1 {end} 
{start} SubPattern2 {end}
{start} {start}SubPattern3{end} {end}
DATA;

preg_match_all('/{start}((?:(?:(?!{(?:start|end)}).)+|(?R))*){end}/', $string, $result);
print_r($result);

Explanation

{start} Match literally
( Start capturing group
- (?: Non capturing group
  - (?: Non capturing group
    - (?!{(?:start|end)}). Negative lookahead to assert what is on the right is not {start} or {end}. If that is the case, match any character.
  - )+|(?R) Close non capturing group and repeat 1+ times or | recurses the entire pattern
- )* Close non capturing group and repeat 0+ times
) Close capturing group
{end} - Match literally

Regex demo

Result:

Array
(
    [0] => Array
        (
            [0] => {start} SubPattern1 {end}
            [1] => {start} SubPattern2 {end}
            [2] => {start} {start}SubPattern3{end} {end}
        )

    [1] => Array
        (
            [0] =>  SubPattern1 
            [1] =>  SubPattern2 
            [2] =>  {start}SubPattern3{end} 
        )

)

Php demo

edited Dec 17 '18 at 08:24

answered Dec 16 '18 at 16:31

The fourth bird

96,715
14
35
52

Can you add explanation about recursive regex ? i am trying my head around it from last half hour. it will be helpful if you can. Thanks in advance – Code Maniac Dec 16 '18 at 16:41
@CodeManiac I have added an explanation. – The fourth bird Dec 16 '18 at 17:03
@Thefourthbird that's works well. But the pattern doesn't match when a curly bracket is present in the sub-pattern. eg: `{start} Pattern{1} {end}` – Jorge Luis Dec 16 '18 at 17:07
@JorgeLuis I have updated my answer. – The fourth bird Dec 17 '18 at 08:19
@Thefourthbird that`s simply perfect! Thanks a lot! – Jorge Luis Dec 17 '18 at 18:12
@JorgeLuis You are welcome, glad it works for your. My advice would be to show in / add to your question what you have tried if in the future you want to ask another question. Good luck! – The fourth bird Dec 17 '18 at 18:49

Difficult PHP Nested Regular Expression

2 Answers2