Allow underscores in big"[float]" (JuliaLang#43346)

base/int.jl

@@ -689,25 +689,30 @@ julia> big"7891.5"
 ```
 """
 macro big_str(s)
+    message = "invalid number format $s for BigInt or BigFloat"
+    throw_error =  :(throw(ArgumentError($message)))
     if '_' in s
         # remove _ in s[2:end-1]
         bf = IOBuffer(maxsize=lastindex(s))
-        print(bf, s[1])
+        c = s[1]
+        print(bf, c)
+        is_prev_underscore = (c == '_')
+        is_prev_dot = (c == '.')
         for c in SubString(s, 2, lastindex(s)-1)
             c != '_' && print(bf, c)
+            c == '_' && is_prev_dot && return throw_error
+            c == '.' && is_prev_underscore && return throw_error
+            is_prev_underscore = (c == '_')
+            is_prev_dot = (c == '.')
         end
         print(bf, s[end])
-        seekstart(bf)
-        n = tryparse(BigInt, String(take!(bf)))
-        n === nothing || return n
-    else
-        n = tryparse(BigInt, s)
-        n === nothing || return n
-        n = tryparse(BigFloat, s)
-        n === nothing || return n
+        s = String(take!(bf))
     end
-    message = "invalid number format $s for BigInt or BigFloat"
-    return :(throw(ArgumentError($message)))
+    n = tryparse(BigInt, s)
+    n === nothing || return n
+    n = tryparse(BigFloat, s)
+    n === nothing || return n
+    return throw_error
 end
 
 ## integer promotions ##

test/int.jl

@@ -324,10 +324,16 @@ end
     end
 end
 
-@testset "issue #21092" begin
+@testset "Underscores in big_str" begin
     @test big"1_0_0_0" == BigInt(1000)
     @test_throws ArgumentError big"1_0_0_0_"
     @test_throws ArgumentError big"_1_0_0_0"
+
+    @test big"1_0.2_5" == BigFloat(10.25)
+    @test_throws ArgumentError big"_1_0.2_5"
+    @test_throws ArgumentError big"1_0.2_5_"
+    @test_throws ArgumentError big"1_0_.2_5"
+    @test_throws ArgumentError big"1_0._2_5"
 end
 
 # issue #26779