regex101: import syntax for regex with grouping

Regular Expression

(?:(?<=@import\s*(['"]|url\()\s*)((?:\S|\\\1)+)\s*(?:\1|\))\s*\)?\s?(.*?);)

New

# Drupal core/tests/Drupal/Tests/Core/Asset/css_test_files/css_input_with_import.css
@import "import1.css";
@import "import2.css";
@import url("http://example.com/style.css");
@import url("//example.com/style.css");

# Examples from https://developer.mozilla.org/en-US/docs/Web/CSS/%40import
@import url("finep;rint.css") print;
@import url("bluish.;css") projection, tv;
@import 'cust;om.css';
@import url("chrome://communi;cator/skin/");
@import "common.css" screen, projection;
@import url('landscape.css') screen and (orientation:landscape);

# Examples using htmlentities
@import 'https://fonts.googleapis.com/css?family=Sedgwick+Ave+Display&amp;subset=latin-ext';
@import "https://fonts.googleapis.com/css?family=Sedgwick+Ave+Display&amp;subset=latin-ext" ;
@import url("https://fonts.googleapis.com/css?family=Sedgwick+Ave+Display&amp;subset=latin-ext");
@import url("https://fonts.googleapis.com/css?family=Sedgwick+Ave+Display&amp;subset=latin-ext") tv;
@import url("https://fonts.googleapis.com/css?family=Sedgwick+Ave+Display&amp;subset=latin-ext") only screen and (max-width:639px) and (orientation: portrait);

# Additional
@import "cu'st;om.css";
@import "cu\"st;om.css";
@import "cu\22st;om.css";
@import 'cus\'t;om.css';
@import 'cus\27t;om.css' ;
@import     'cus(t;om.css'  ;
@import 'cus)t;om.css';
@import 'cus(t);om.css';

@import url(      "     http://user:pass@example.com/c'u(s)t;o\"m.css    "     )    ;
@import    url(      '     http://user:pass@example.com/c"u(s)t;o\'m.css    '     )   ;
@import    url(      http://user:pass@example.com/c\'u\(s\)t;om.css        )   ;


# Inline
@import "cu'st;om.css"; @import "cu\"st;om.css"; .my-class { display: block; }

@import "cu'st;om.css";@import url(      "     http://user:pass@example.com/c'u(s)t;o\"m.css    "     )    ;.some-class { color: rgba(0, 0, 0); }@import    url(      http://user:pass@example.com/c\'u\(s\)t;om.css        )   ;.some-class { color: rgba(0, 0, 0); }

Extraction

# Drupal core/tests/Drupal/Tests/Core/Asset/css_test_files/css_input_with_import.css
@import "import1.css";
@import "import2.css";
@import url("http://example.com/style.css");
@import url("//example.com/style.css");

# Examples from https://developer.mozilla.org/en-US/docs/Web/CSS/%40import
@import url("finep;rint.css") print;
@import url("bluish.;css") projection, tv;
@import 'cust;om.css';
@import url("chrome://communi;cator/skin/");
@import "common.css" screen, projection;
@import url('landscape.css') screen and (orientation:landscape);

# Examples using htmlentities
@import 'https://fonts.googleapis.com/css?family=Sedgwick+Ave+Display&amp;subset=latin-ext';
@import "https://fonts.googleapis.com/css?family=Sedgwick+Ave+Display&amp;subset=latin-ext" ;
@import url("https://fonts.googleapis.com/css?family=Sedgwick+Ave+Display&amp;subset=latin-ext");
@import url("https://fonts.googleapis.com/css?family=Sedgwick+Ave+Display&amp;subset=latin-ext") tv;
@import url("https://fonts.googleapis.com/css?family=Sedgwick+Ave+Display&amp;subset=latin-ext") only screen and (max-width:639px) and (orientation: portrait);

# Additional
@import "cu'st;om.css";
@import "cu\"st;om.css";
@import "cu\22st;om.css";
@import 'cus\'t;om.css';
@import 'cus\27t;om.css' ;
@import     'cus(t;om.css'  ;
@import 'cus)t;om.css';
@import 'cus(t);om.css';

@import url(      "     http://user:pass@example.com/c'u(s)t;o\"m.css    "     )    ;
@import    url(      '     http://user:pass@example.com/c"u(s)t;o\'m.css    '     )   ;
@import    url(      http://user:pass@example.com/c\'u\(s\)t;om.css        )   ;


# Inline
@import "cu'st;om.css"; @import "cu\"st;om.css"; .my-class { display: block; }

@import "cu'st;om.css";@import url(      "     http://user:pass@example.com/c'u(s)t;o\"m.css    "     )    ;.some-class { color: rgba(0, 0, 0); }@import    url(      http://user:pass@example.com/c\'u\(s\)t;om.css        )   ;.some-class { color: rgba(0, 0, 0); }