syntax: yaml: colorize backslash-escaped characters as valid or invalid

See https://yaml.org/spec/1.2.2/#57-escaped-characters.

Also prevent an escaped double quote from breaking the coloring of
a string, and allow hyphens in an anchor.

syntax/yaml.nanorc

@@ -16,13 +16,13 @@ color lightgreen "\[(\w|::|[, -])+\]:( |$)"
 # Values (booleans, numbers, octal/hex, dates, strings):
 color lightmagenta "[:,] +(Y(es)?|No?|y(es)?|no?|[Tt]rue|[Ff]alse|[Oo](n|ff))( *[]}]|, | +#|$)"
 color lightmagenta "[:,] +[+-]?[0-9]+(\.([0-9]+)?)?( *[]}]|, | +#|$)"
-color lightmagenta " 0(o[0-7]+|x[0-9ABCDEFabcdef]+)( *[]}]|, | +#|$)"
+color lightmagenta " 0(o[0-7]+|x[[:xdigit:]]+)( *[]}]|, | +#|$)"
 color normal "[:,]( |$)"
 color lightmagenta " [12][0-9]{3}-(0[1-9]|1[0-2])-(0[1-9]|[12][0-9]|3[01])"
-color lightmagenta "("[^"]+"|'[^']+')"
+color lightmagenta "("([^"]|\\")+"|'[^']+')"
 
 # Anchors and references:
-color pink " [&*]\w+( |$)"
+color pink " [&*](\w|-)+( |$)"
 
 # Symbols:
 color bold,lagoon "^(---|\.\.\.)( |$)" " [|>]([1-9]?[+-]|[+-][1-9]?)?$"
@@ -32,6 +32,11 @@ color yellow "(^ *- |[]{}[])"
 color mint " !!(binary|bool|float|int|map|null|omap|seq|set|str)( |$)"
 color mint " !(\w|-)+( |$)"
 
+# Escaped characters (first color all as bad, then recolor the good ones):
+color lightwhite,red "\\."
+color orange "\\([0abefnrtv"/ \_NLP]|$)"
+color orange "\\(x[[:xdigit:]]{2}|u[[:xdigit:]]{4}|U[[:xdigit:]]{8})"
+
 # Mistakes (control codes, trailing space):
 color ,red "[[:cntrl:]]| +$"