regex - How can I replace the Nth instance of a string in PHP? -

i need dynamically add active class shortcode in wordpress based on active number set in it's parent shortcode.

the parent looks this:

$active = 3; $output .= "\n\t\t".wpb_js_remove_wpautop($content); 

the $content variable can technically consist of in between shortcodes tags, specifically, should include more child shortcodes (it's tabs shortcode parent tabs , children tab).

what need parse $content , replace nth ($active variable) instance of [vc_tab [vc_tab active="true"

i realize there better ways design shortcode compensate limited in can because modifying visual composer use bootstrap tabs instead of jquery , bootstrap tabs need active class on both <li> , <div class="tab-pane"> elements don't want users have add active number parent shortcode , active true/false child confusing.

so far googling can find replacing first occurrence or occurrences, not nth one.

temporarily using jquery make appropriate <div> active results in undesirable fouc there no pane visible on load , 1 pops place when jquery runs.

$('.tabs-wrap').each(function(){   var activetab = $(this).find('.tabbed li.active a').attr('href');   $(this).find(activetab).addclass('active'); });   

this interesting problem, , got me thinking if need regex this.

my conclusion don't believe do. need find occurrence of nth $needle, , replace replacement. can achieved through simple string manipulation, follows:

1. define variables. input string, $active index, $needle looking for, , $replacement replace nth match with.

   $string  = "[vc_tab [vc_tab [vc_tab [vc_tab [vc_tab [vc_tab"; $active  = 3; $needle  = "[vc_tab"; $replace = '[vc_tab active="true"'; 

2. now, need make sure there enough $needles in $string, otherwise can't replacement.

   if( substr_count( $string, $needle) < $active) {     throw new exception("there aren't enough needles in string replacement"); } 

3. now, know can replacement. let's find $index in string nth occurrence of $needle ends:

   $count = 0; $index = 0; while( $count++ < $active) {     $index = strpos( $string, $needle, $index) + strlen( $needle); } 

4. now, $index pointing here in string:

   [vc_tab [vc_tab [vc_tab [vc_tab [vc_tab [vc_tab                        ^                        here 

   we can simple substr()'s form final string:

   $result = substr( $string, 0, $index - strlen( $needle)) . $replace . substr( $string, $index);           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^              ^^^^^^^^^^^^^^^^^^^^^^^^           before nth match                                        after nth match 

   so, end concatenating:

   • [vc_tab [vc_tab
   • [vc_tab active="true"
   • [vc_tab [vc_tab [vc_tab

this results in our final output:

[vc_tab [vc_tab [vc_tab active="true" [vc_tab [vc_tab [vc_tab 

try out in the demo!