regex101: PCRE (Legacy) Match Workspace

Regular Expression

@font-face\s*    # Match @font-face and some spaces
(                # Start group 1
   \{            # Match {
   (?:           # A non-capturing group
      [^{}]+     # Match anything except {} one or more times
      |          # Or
      (?1)       # Recurse/rerun the expression of group 1
   )*            # Repeat 0 or more times
   \}            # Match }
)                # End group 1
(*SKIP)(*FAIL)   # Skip it
|                # Or
url\s*\(         # Match url, optionally some whitespaces and then (
\s*              # Match optionally some whitespaces
("|'|)           # It seems that the quotes are optional according to http://www.w3.org/TR/CSS2/syndata.html#uri
(?!["']?(?:https?://|ftp://))  # Put your negative-rules here (do not match url's with http, https or ftp)
(?:[^\\]|\\.)*?  # Match anything except a backslash or backslash and a character zero or more times ungreedy
\2               # Match what was matched in group 2
\s*              # Match optionally some whitespaces
\)               # Match )

New

.someclass0 {
  background: url('http://images/someimage.png') no-repeat;
}

@font-face {
  font-family: 'FontAwesome';
  src: url("fonts/fontawesome-webfont.eot?v=4.0.3");
  src: url("fonts/fontawesome-webfont.eot?#iefix&v=4.0.3") format("embedded-opentype"), url("fonts/fontawesome-webfont.woff?v=4.0.3") format("woff"), url("fonts/fontawesome-webfont.ttf?v=4.0.3") format("truetype"), url("fonts/fontawesome-webfont.svg?v=4.0.3#fontawesomeregular") format("svg");
  font-weight: normal;
  font-style: normal;
}

.someclass1 {
  background: url('images/someimage.png') no-repeat;
}

.someclass2 {
  background: url("images/someimage.png") no-repeat;
}

.someclass3 {
  background: url(images/someimage.png) no-repeat;
}

Substitution

.someclass0 {
  background: url('http://images/someimage.png') no-repeat;
}

@font-face {
  font-family: 'FontAwesome';
  src: url("fonts/fontawesome-webfont.eot?v=4.0.3");
  src: url("fonts/fontawesome-webfont.eot?#iefix&v=4.0.3") format("embedded-opentype"), url("fonts/fontawesome-webfont.woff?v=4.0.3") format("woff"), url("fonts/fontawesome-webfont.ttf?v=4.0.3") format("truetype"), url("fonts/fontawesome-webfont.svg?v=4.0.3#fontawesomeregular") format("svg");
  font-weight: normal;
  font-style: normal;
}

.someclass1 {
  background: url('images/someimage.png') no-repeat;
}

.someclass2 {
  background: url("images/someimage.png") no-repeat;
}

.someclass3 {
  background: url(images/someimage.png) no-repeat;
}