fix #22926, raw string escaping rules that allow writing all strings

NEWS.md

@@ -130,6 +130,10 @@ Language changes
   * Like `_`, variable names consisting only of underscores can be assigned,
     but accessing their values is deprecated ([#24221]).
 
+  * Raw string literal escaping rules have been changed to make it possible to write all strings.
+    The rule is that backslashes escape both quotes and other backslashes, but only when appearing
+    directly before a quote character ([#22926]).
+
 Breaking changes
 ----------------
 

base/strings/io.jl

@@ -371,15 +371,23 @@ macro b_str(s); :(Vector{UInt8}($(unescape_string(s)))); end
     @raw_str -> String
 
 Create a raw string without interpolation and unescaping.
-The exception is that quotation marks still must be escaped.
+The exception is that quotation marks still must be escaped. Backslashes
+escape both question marks and other backslashes, but only when immediately
+preceding a quote character.
 
 # Examples
 ```jldoctest
-julia> raw"\\""
-"\\""
+julia> println(raw"\\ \$x")
+\\ \$x
 
-julia> raw""\" " ""\"
-" \\" "
+julia> println(raw"\\"")
+"
+
+julia> println(raw"\\\\\\"")
+\\"
+
+julia> println(raw"\\\\x \\\\\\"")
+\\\\x \\"
 ```
 """
 macro raw_str(s); s; end

doc/src/manual/strings.md

@@ -920,5 +920,19 @@ non-standard string literals of the form `raw"..."`. Raw string literals create
 ordinary `String` objects which contain the enclosed contents exactly as
 entered with no interpolation or unescaping. This is useful for strings which
 contain code or markup in other languages which use `$` or `\` as special
-characters. The exception is quotation marks that still must be
-escaped, e.g. `raw"\""` is equivalent to `"\""`.
+characters.
+
+The exception is that quotation marks still must be escaped, e.g. `raw"\""` is equivalent
+to `"\""`.
+To make it possible to express all strings, backslashes then also must be escaped, but
+only when appearing right before a quote character:
+
+```jldoctest
+julia> println(raw"\\ \\\"")
+\\ \"
+```
+
+Notice that the first two backslashes appear verbatim in the output, since they do not
+precede a quote character.
+However, the next backslash character escapes the backslash that follows it, and the
+last backslash escapes a quote, since these backslashes appear before a quote.

src/julia-parser.scm

@@ -2107,12 +2107,26 @@
        (loop c b e 0))
 
       ((eqv? c #\\)
-       (let ((nxch (not-eof-for delim (read-char p))))
-         (if (or (not raw)
-                 (not (or (eqv? nxch delim) #;(eqv? nxch #\\))))
-             (write-char #\\ b))
-         (write-char nxch b)
-         (loop (read-char p) b e 0)))
+       (if raw
+           (let backslashes ((count 1)
+                             (nxch (not-eof-for delim (read-char p))))
+             (cond ((eqv? nxch #\\)
+                    (backslashes (+ 1 count)
+                                 (not-eof-for delim (read-char p))))
+                   ((eqv? nxch delim)
+                    (io.write b (string.rep "\\" (div count 2)))
+                    (if (odd? count)
+                        (begin (write-char delim b)
+                               (loop (read-char p) b e 0))
+                        (loop nxch b e 0)))
+                   (else
+                    (io.write b (string.rep "\\" count))
+                    (write-char nxch b)
+                    (loop (read-char p) b e 0))))
+           (let ((nxch (not-eof-for delim (read-char p))))
+             (write-char #\\ b)
+             (write-char nxch b)
+             (loop (read-char p) b e 0))))
 
       ((and (eqv? c #\$) (not raw))
        (let ((ex (parse-interpolate s)))

test/strings/io.jl

@@ -183,23 +183,7 @@ end
     @test Base.unindent("\n\t\n    \tfoo",4) == "\n    \n    foo"
     @test Base.unindent("\n\tfoo\tbar",4) == "\n    foo     bar"
 end
-@testset "raw_str macro" begin
-    @test raw"$" == "\$"
-    @test raw"\n" == "\\n"
-    @test raw"\t" == "\\t"
 
-    s1 = raw"""
-         lorem ipsum\n
-         $x = 1$
-         """
-
-    s2 = """
-         lorem ipsum\\n
-         \$x = 1\$
-         """
-
-    @test s1 == s2
-end
 # issue #22021, string realloc bug with join
 s22021 = String["\"\"\"
      non_max_suppression(boxes, scores, max_output_size; iou_threshold=nothing)

test/syntax.jl

@@ -1171,3 +1171,30 @@ end
 # recursively calling expand-forms.
 @test [(0,0)... 1] == [0 0 1]
 @test Float32[(0,0)... 1] == Float32[0 0 1]
+
+@testset "raw_str macro" begin
+    @test raw"$" == "\$"
+    @test raw"\n" == "\\n"
+    @test raw"\t" == "\\t"
+
+    s1 = raw"""
+         lorem ipsum\n
+         $x = 1$
+         """
+
+    s2 = """
+         lorem ipsum\\n
+         \$x = 1\$
+         """
+
+    @test s1 == s2
+
+    # issue #22926
+    @test raw"\\" == "\\"
+    @test raw"\\\\" == "\\\\"
+    @test raw "\"" == "\""
+    @test raw"\\\"" == "\\\""
+    @test raw"\\x\\" == "\\\\x\\"
+    @test raw"x \\\" y" == "x \\\" y"
+    @test raw"x \\\ y" == "x \\\\\\ y"
+end