Improve JSON output when there is leading data before the actual JSON body
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 847e70b519..ec56074ab6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ## [2.6.0.dev0](https://github.com/httpie/httpie/compare/2.5.0...master) (unreleased)
 
+- Added support for formatting & coloring of JSON bodies preceded by non-JSON data (e.g., an XXSI prefix). ([#1130](https://github.com/httpie/httpie/issues/1130))
+
 ## [2.5.0](https://github.com/httpie/httpie/compare/2.4.0...2.5.0) (2021-09-06)
 
 Blog post: [What’s new in HTTPie 2.5.0](https://httpie.io/blog/httpie-2.5.0)
diff --git a/httpie/output/formatters/colors.py b/httpie/output/formatters/colors.py
index ff182d2387..d0187337b0 100644
--- a/httpie/output/formatters/colors.py
+++ b/httpie/output/formatters/colors.py
@@ -9,10 +9,12 @@
 from pygments.formatters.terminal import TerminalFormatter
 from pygments.formatters.terminal256 import Terminal256Formatter
 from pygments.lexer import Lexer
+from pygments.lexers.data import JsonLexer
 from pygments.lexers.special import TextLexer
 from pygments.lexers.text import HttpLexer as PygmentsHttpLexer
 from pygments.util import ClassNotFound
 
+from ..lexers.json import EnhancedJsonLexer
 from ...compat import is_windows
 from ...context import Environment
 from ...plugins import FormatterPlugin
@@ -60,6 +62,7 @@ def __init__(
             http_lexer = PygmentsHttpLexer()
             formatter = TerminalFormatter()
         else:
+            from ..lexers.http import SimplifiedHTTPLexer
             http_lexer = SimplifiedHTTPLexer()
             formatter = Terminal256Formatter(
                 style=self.get_style_class(color_scheme)
@@ -151,55 +154,12 @@ def get_lexer(
         else:
             lexer = pygments.lexers.get_lexer_by_name('json')
 
-    return lexer
-
-
-class SimplifiedHTTPLexer(pygments.lexer.RegexLexer):
-    """Simplified HTTP lexer for Pygments.
-
-    It only operates on headers and provides a stronger contrast between
-    their names and values than the original one bundled with Pygments
-    (:class:`pygments.lexers.text import HttpLexer`), especially when
-    Solarized color scheme is used.
+    # Use our own JSON lexer: it supports JSON bodies preceded by non-JSON data
+    # as well as legit JSON bodies.
+    if isinstance(lexer, JsonLexer):
+        lexer = EnhancedJsonLexer()
 
-    """
-    name = 'HTTP'
-    aliases = ['http']
-    filenames = ['*.http']
-    tokens = {
-        'root': [
-            # Request-Line
-            (r'([A-Z]+)( +)([^ ]+)( +)(HTTP)(/)(\d+\.\d+)',
-             pygments.lexer.bygroups(
-                 pygments.token.Name.Function,
-                 pygments.token.Text,
-                 pygments.token.Name.Namespace,
-                 pygments.token.Text,
-                 pygments.token.Keyword.Reserved,
-                 pygments.token.Operator,
-                 pygments.token.Number
-             )),
-            # Response Status-Line
-            (r'(HTTP)(/)(\d+\.\d+)( +)(\d{3})( +)(.+)',
-             pygments.lexer.bygroups(
-                 pygments.token.Keyword.Reserved,  # 'HTTP'
-                 pygments.token.Operator,  # '/'
-                 pygments.token.Number,  # Version
-                 pygments.token.Text,
-                 pygments.token.Number,  # Status code
-                 pygments.token.Text,
-                 pygments.token.Name.Exception,  # Reason
-             )),
-            # Header
-            (r'(.*?)( *)(:)( *)(.+)', pygments.lexer.bygroups(
-                pygments.token.Name.Attribute,  # Name
-                pygments.token.Text,
-                pygments.token.Operator,  # Colon
-                pygments.token.Text,
-                pygments.token.String  # Value
-            ))
-        ]
-    }
+    return lexer
 
 
 class Solarized256Style(pygments.style.Style):
diff --git a/httpie/output/formatters/json.py b/httpie/output/formatters/json.py
index 65cbcd1989..bc6151e42f 100644
--- a/httpie/output/formatters/json.py
+++ b/httpie/output/formatters/json.py
@@ -17,15 +17,16 @@ def format_body(self, body: str, mime: str) -> str:
         ]
         if (self.kwargs['explicit_json']
                 or any(token in mime for token in maybe_json)):
+            from ..utils import load_prefixed_json
             try:
-                obj = json.loads(body)
+                data_prefix, json_obj = load_prefixed_json(body)
             except ValueError:
                 pass  # Invalid JSON, ignore.
             else:
                 # Indent, sort keys by name, and avoid
                 # unicode escapes to improve readability.
-                body = json.dumps(
-                    obj=obj,
+                body = data_prefix + json.dumps(
+                    obj=json_obj,
                     sort_keys=self.format_options['json']['sort_keys'],
                     ensure_ascii=False,
                     indent=self.format_options['json']['indent']
diff --git a/httpie/output/lexers/__init__.py b/httpie/output/lexers/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/httpie/output/lexers/http.py b/httpie/output/lexers/http.py
new file mode 100644
index 0000000000..4c2b00d252
--- /dev/null
+++ b/httpie/output/lexers/http.py
@@ -0,0 +1,49 @@
+import pygments
+
+
+class SimplifiedHTTPLexer(pygments.lexer.RegexLexer):
+    """Simplified HTTP lexer for Pygments.
+
+    It only operates on headers and provides a stronger contrast between
+    their names and values than the original one bundled with Pygments
+    (:class:`pygments.lexers.text import HttpLexer`), especially when
+    Solarized color scheme is used.
+
+    """
+    name = 'HTTP'
+    aliases = ['http']
+    filenames = ['*.http']
+    tokens = {
+        'root': [
+            # Request-Line
+            (r'([A-Z]+)( +)([^ ]+)( +)(HTTP)(/)(\d+\.\d+)',
+             pygments.lexer.bygroups(
+                 pygments.token.Name.Function,
+                 pygments.token.Text,
+                 pygments.token.Name.Namespace,
+                 pygments.token.Text,
+                 pygments.token.Keyword.Reserved,
+                 pygments.token.Operator,
+                 pygments.token.Number
+             )),
+            # Response Status-Line
+            (r'(HTTP)(/)(\d+\.\d+)( +)(\d{3})( +)(.+)',
+             pygments.lexer.bygroups(
+                 pygments.token.Keyword.Reserved,  # 'HTTP'
+                 pygments.token.Operator,  # '/'
+                 pygments.token.Number,  # Version
+                 pygments.token.Text,
+                 pygments.token.Number,  # Status code
+                 pygments.token.Text,
+                 pygments.token.Name.Exception,  # Reason
+             )),
+            # Header
+            (r'(.*?)( *)(:)( *)(.+)', pygments.lexer.bygroups(
+                pygments.token.Name.Attribute,  # Name
+                pygments.token.Text,
+                pygments.token.Operator,  # Colon
+                pygments.token.Text,
+                pygments.token.String  # Value
+            ))
+        ]
+    }
diff --git a/httpie/output/lexers/json.py b/httpie/output/lexers/json.py
new file mode 100644
index 0000000000..a235c4f3a3
--- /dev/null
+++ b/httpie/output/lexers/json.py
@@ -0,0 +1,31 @@
+import re
+
+from pygments.lexer import bygroups, using, RegexLexer
+from pygments.lexers.data import JsonLexer
+from pygments.token import Token
+
+PREFIX_TOKEN = Token.Error
+PREFIX_REGEX = r'[^{\["]+'
+
+
+class EnhancedJsonLexer(RegexLexer):
+    """
+    Enhanced JSON lexer for Pygments.
+
+    It adds support for eventual data prefixing the actual JSON body.
+
+    """
+    name = 'JSON'
+    flags = re.IGNORECASE | re.DOTALL
+    tokens = {
+        'root': [
+            # Eventual non-JSON data prefix followed by actual JSON body.
+            # FIX: data prefix + number (integer or float) are not correctly handled.
+            (
+                fr'({PREFIX_REGEX})' + r'((?:[{\["]|true|false|null).+)',
+                bygroups(PREFIX_TOKEN, using(JsonLexer))
+            ),
+            # JSON body.
+            (r'.+', using(JsonLexer)),
+        ],
+    }
diff --git a/httpie/output/utils.py b/httpie/output/utils.py
new file mode 100644
index 0000000000..5ae7f603d5
--- /dev/null
+++ b/httpie/output/utils.py
@@ -0,0 +1,36 @@
+import json
+import re
+from typing import Tuple
+
+from .lexers.json import PREFIX_REGEX
+
+
+def load_prefixed_json(data: str) -> Tuple[str, json.JSONDecoder]:
+    """Simple JSON loading from `data`.
+
+    """
+    # First, the full data.
+    try:
+        return '', json.loads(data)
+    except ValueError:
+        pass
+
+    # Then, try to find the start of the actual body.
+    data_prefix, body = parse_prefixed_json(data)
+    try:
+        return data_prefix, json.loads(body)
+    except ValueError:
+        raise ValueError('Invalid JSON')
+
+
+def parse_prefixed_json(data: str) -> Tuple[str, str]:
+    """Find the potential JSON body from `data`.
+
+    Sometimes the JSON body is prefixed with a XSSI magic string, specific to the server.
+    Return a tuple (data prefix, actual JSON body).
+
+    """
+    matches = re.findall(PREFIX_REGEX, data)
+    data_prefix = matches[0] if matches else ''
+    body = data[len(data_prefix):]
+    return data_prefix, body
diff --git a/tests/test_json.py b/tests/test_json.py
new file mode 100644
index 0000000000..4d210f23e0
--- /dev/null
+++ b/tests/test_json.py
@@ -0,0 +1,40 @@
+import json
+
+import pytest
+import responses
+
+from httpie.cli.constants import PRETTY_MAP
+from httpie.compat import is_windows
+from httpie.output.formatters.colors import ColorFormatter
+
+from .utils import MockEnvironment, http, URL_EXAMPLE
+
+TEST_JSON_XXSI_PREFIXES = (r")]}',\n", ")]}',", 'while(1);', 'for(;;)', ')', ']', '}')
+TEST_JSON_VALUES = ({}, {'a': 0, 'b': 0}, [], ['a', 'b'], 'foo', True, False, None)  # FIX: missing int & float
+TEST_PREFIX_TOKEN_COLOR = '\x1b[38;5;15m' if is_windows else '\x1b[04m\x1b[91m'
+
+
+@pytest.mark.parametrize('data_prefix', TEST_JSON_XXSI_PREFIXES)
+@pytest.mark.parametrize('json_data', TEST_JSON_VALUES)
+@pytest.mark.parametrize('pretty', PRETTY_MAP.keys())
+@responses.activate
+def test_json_formatter_with_body_preceded_by_non_json_data(data_prefix, json_data, pretty):
+    """Test JSON bodies preceded by non-JSON data."""
+    body = data_prefix + json.dumps(json_data)
+    content_type = 'application/json'
+    responses.add(responses.GET, URL_EXAMPLE, body=body,
+                  content_type=content_type)
+
+    colored_output = pretty in ('all', 'colors')
+    env = MockEnvironment(colors=256) if colored_output else None
+    r = http('--pretty=' + pretty, URL_EXAMPLE, env=env)
+
+    indent = None if pretty in ('none', 'colors') else 4
+    expected_body = data_prefix + json.dumps(json_data, indent=indent)
+    if colored_output:
+        fmt = ColorFormatter(env, format_options={'json': {'format': True, 'indent': 4}})
+        expected_body = fmt.format_body(expected_body, content_type)
+        # Check to ensure the non-JSON data prefix is colored only one time,
+        # meaning it was correctly handled as a whole.
+        assert TEST_PREFIX_TOKEN_COLOR + data_prefix in expected_body, expected_body
+    assert expected_body in r

In some special cases, to prevent against Cross Site Script Inclusion (XSSI) attacks, the JSON response body starts with a magic prefix line that must be stripped before feeding the rest of the response body to the JSON parser.
Such prefix is now simply ignored from the parser but still printed in the terminal.

Supersedes #529.