F#: Chars can only contain one character (#1570)

This PR fixes #1480.

components/prism-fsharp.js

@@ -11,7 +11,7 @@ Prism.languages.fsharp = Prism.languages.extend('clike', {
 	],
 	'keyword': /\b(?:let|return|use|yield)(?:!\B|\b)|\b(abstract|and|as|assert|base|begin|class|default|delegate|do|done|downcast|downto|elif|else|end|exception|extern|false|finally|for|fun|function|global|if|in|inherit|inline|interface|internal|lazy|match|member|module|mutable|namespace|new|not|null|of|open|or|override|private|public|rec|select|static|struct|then|to|true|try|type|upcast|val|void|when|while|with|asr|land|lor|lsl|lsr|lxor|mod|sig|atomic|break|checked|component|const|constraint|constructor|continue|eager|event|external|fixed|functor|include|method|mixin|object|parallel|process|protected|pure|sealed|tailcall|trait|virtual|volatile)\b/,
 	'string': {
-		pattern: /(?:"""[\s\S]*?"""|@"(?:""|[^"])*"|("|')(?:\\[\s\S]|(?!\1)[^\\])*\1)B?/,
+		pattern: /(?:"""[\s\S]*?"""|@"(?:""|[^"])*"|"(?:\\[\s\S]|[^\\"])*")B?|'(?:[^\\']|\\.)'B?/,
 		greedy: true
 	},
 	'number': [

tests/languages/fsharp/issue1480.test

@@ -0,0 +1,37 @@
+let foo' = failWith "foo"
+
+let bar' = failWith "bar"
+
+let map (f: 'a -> 'b) (xs: 'a list): 'b list = failWith "not implemented"
+
+----------------------------------------------------
+
+[
+	["keyword", "let"], " foo' ", ["operator", "="], " failWith ", ["string", "\"foo\""],
+	["keyword", "let"], " bar' ", ["operator", "="], " failWith ", ["string", "\"bar\""],
+
+	["keyword", "let"],
+	" map ",
+	["punctuation", "("],
+	"f",
+	["punctuation", ":"],
+	" 'a ",
+	["operator", "-"],
+	["operator", ">"],
+	" 'b",
+	["punctuation", ")"],
+	["punctuation", "("],
+	"xs",
+	["punctuation", ":"],
+	" 'a list",
+	["punctuation", ")"],
+	["punctuation", ":"],
+	" 'b list ",
+	["operator", "="],
+	" failWith ",
+	["string", "\"not implemented\""]
+]
+
+----------------------------------------------------
+
+Checks for apostrophes in names. See #1480.

tests/languages/fsharp/string_feature.test

@@ -15,6 +15,7 @@ bar"""
 """foo"""B
 
 'a'
+'a'B
 '\''
 '\\'
 
@@ -36,6 +37,7 @@ bar"""
 	["string", "\"\"\"foo\"\"\"B"],
 
 	["string", "'a'"],
+	["string", "'a'B"],
 	["string", "'\\''"],
 	["string", "'\\\\'"]
 ]