Fix issue with special keys that require quoting in mappings

src/writer.jl

@@ -73,9 +73,21 @@ end
 function _print(io::IO, pair::Pair, level::Int=0, ignore_level::Bool=false)
     key = if pair[1] === nothing
         "null" # this is what the YAML parser interprets as 'nothing'
+    elseif pair[1] isa AbstractString && (
+        occursin('#', pair[1]) ||
+            any(
+                c -> startswith(pair[1], c),
+                [
+                    '{', '}', '[', ']', '&', '*', '#', '?', '|', '-', '<', '>', '=',
+                    '!', '%', '@', ':', '`', ',', '"', '\'',
+                ],
+            )
+        )
+        string("\"", escape_string(pair[1]), "\"") # special keys that require quoting
     else
         string(pair[1]) # any useful case
     end
+
     print(io, _indent(key * ":", level, ignore_level)) # print the key
     if (pair[2] isa AbstractDict || pair[2] isa AbstractVector) && !isempty(pair[2])
         print(io, "\n") # a line break is needed before a recursive structure

test/runtests.jl

@@ -491,4 +491,14 @@ end
     @test collect(YAML.load_all(input)) == expected
 end
 
+# issue #236 - special string keys that require enclosure in quotes
+@testset "issue #236" begin
+    for char in [
+        "{", "}", "[", "]", "&", "*", "#", "?", "|", "-", "<", ">",
+        "=", "!", "%", "@", ":", "`", ",", "'", "\"",
+    ]
+        @test YAML.load(YAML.yaml(Dict(string(char) => 0.0))) == Dict(char => 0.0)
+    end
+end
+
 end  # module