From c6617accc59e3f80a0aa893bbdc7242d045720af Mon Sep 17 00:00:00 2001
From: Tobi Lutke <tobi@shopify.com>
Date: Wed, 11 Mar 2026 08:52:49 -0400
Subject: [PATCH] replace SIMPLE_CONDITION regex with manual byte parser in
 if/elsif lax_parse

---
 lib/liquid/tags/if.rb | 116 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 109 insertions(+), 7 deletions(-)

diff --git a/lib/liquid/tags/if.rb b/lib/liquid/tags/if.rb
index 45e85332..d05b1ef2 100644
--- a/lib/liquid/tags/if.rb
+++ b/lib/liquid/tags/if.rb
@@ -88,19 +88,121 @@ module Liquid
     # Fast path regex for simple conditions: "expr", "expr op expr" (no and/or)
     SIMPLE_CONDITION = /\A\s*(#{QuotedFragment})\s*(?:([=!<>a-z_]+)\s*(#{QuotedFragment}))?\s*\z/o
 
+    # Operators indexed by first byte for fast lookup
+    COMPARISON_OPS = {
+      '==' => '==', '!=' => '!=', '<>' => '<>',
+      '<=' => '<=', '>=' => '>=', '<' => '<', '>' => '>',
+      'contains' => 'contains',
+    }.freeze
+
+    # Parse a simple condition "expr [op expr]" without regex.
+    # Returns [left, op, right] or nil if not parseable.
+    def self.parse_simple_condition(markup)
+      len = markup.bytesize
+      pos = 0
+
+      # Skip leading whitespace
+      pos += 1 while pos < len && (b = markup.getbyte(pos)) && (b == 32 || b == 9 || b == 10 || b == 13)
+      return nil if pos >= len
+
+      # Scan left expression (QuotedFragment): quoted string or non-whitespace/comma/pipe sequence
+      left_start = pos
+      b = markup.getbyte(pos)
+      if b == 34 || b == 39 # quoted string
+        quote = b
+        pos += 1
+        pos += 1 while pos < len && markup.getbyte(pos) != quote
+        pos += 1 if pos < len # closing quote
+      else
+        # Non-whitespace, non-comma, non-pipe chars (QuotedFragment without quotes)
+        while pos < len
+          b = markup.getbyte(pos)
+          break if b == 32 || b == 9 || b == 10 || b == 13 || b == 44 || b == 124 # space, tab, \n, \r, comma, pipe
+          pos += 1
+        end
+      end
+      left_end = pos
+
+      return nil if left_start == left_end
+
+      # Skip whitespace
+      pos += 1 while pos < len && (b = markup.getbyte(pos)) && (b == 32 || b == 9 || b == 10 || b == 13)
+
+      # End of markup? Simple truthiness
+      if pos >= len
+        left = markup.byteslice(left_start, left_end - left_start)
+        return [left, nil, nil]
+      end
+
+      # Scan operator
+      op_start = pos
+      b = markup.getbyte(pos)
+      if b == 61 || b == 33 || b == 60 || b == 62 # =, !, <, >
+        pos += 1
+        b2 = markup.getbyte(pos)
+        pos += 1 if b2 && (b2 == 61 || b2 == 62) # second char of ==, !=, <=, >=, <>
+      elsif b == 99 # 'c' for 'contains'
+        while pos < len
+          b = markup.getbyte(pos)
+          break unless (b >= 97 && b <= 122) || (b >= 65 && b <= 90) || b == 95
+          pos += 1
+        end
+      else
+        return nil # unknown operator start
+      end
+      op = markup.byteslice(op_start, pos - op_start)
+      return nil unless COMPARISON_OPS.key?(op)
+      op = COMPARISON_OPS[op] # use frozen string
+
+      # Skip whitespace
+      pos += 1 while pos < len && (b = markup.getbyte(pos)) && (b == 32 || b == 9 || b == 10 || b == 13)
+      return nil if pos >= len # op without right operand
+
+      # Scan right expression
+      right_start = pos
+      b = markup.getbyte(pos)
+      if b == 34 || b == 39
+        quote = b
+        pos += 1
+        pos += 1 while pos < len && markup.getbyte(pos) != quote
+        pos += 1 if pos < len
+      else
+        while pos < len
+          b = markup.getbyte(pos)
+          break if b == 32 || b == 9 || b == 10 || b == 13 || b == 44 || b == 124
+          pos += 1
+        end
+      end
+      right_end = pos
+
+      return nil if right_start == right_end
+
+      # Skip trailing whitespace
+      pos += 1 while pos < len && (b = markup.getbyte(pos)) && (b == 32 || b == 9 || b == 10 || b == 13)
+      return nil unless pos >= len # extra stuff after right expr
+
+      left = markup.byteslice(left_start, left_end - left_start)
+      right = markup.byteslice(right_start, right_end - right_start)
+      [left, op, right]
+    end
+
     def lax_parse(markup)
       # Fastest path: simple identifier truthiness like "product.available" or "forloop.first"
       if (simple = Variable.simple_variable_markup(markup))
         return Condition.new(parse_expression(simple))
       end
 
-      # Fast path: simple condition without and/or
-      if !markup.include?(' and ') && !markup.include?(' or ') && markup =~ SIMPLE_CONDITION
-        return Condition.new(
-          parse_expression(Regexp.last_match(1)),
-          Regexp.last_match(2),
-          Regexp.last_match(3) ? parse_expression(Regexp.last_match(3)) : nil,
-        )
+      # Fast path: simple condition without and/or — manual byte parser
+      if !markup.include?(' and ') && !markup.include?(' or ')
+        parsed = If.parse_simple_condition(markup)
+        if parsed
+          left, op, right = parsed
+          return Condition.new(
+            parse_expression(left),
+            op,
+            right ? parse_expression(right) : nil,
+          )
+        end
       end
 
       expressions = markup.scan(ExpressionsAndOperators)