also escape leading ascii letters

Author: bakkot

spec.emu

@@ -31,9 +31,10 @@ contributors:
           1. Let _escaped_ be the empty String.
           1. Let _cpList_ be StringToCodePoints(_S_).
           1. For each code point _c_ in _cpList_, do
-            1. If _escaped_ is the empty String and _c_ is matched by |DecimalDigit|, then
-              1. NOTE: Escaping a leading digit ensures that output corresponds with pattern text which may be used after a `\0` character escape or a |DecimalEscape| such as `\1` and still match _S_ rather than be interpreted as an extension of the preceding escape sequence.
-              1. Set _escaped_ to the string-concatenation of _escaped_, the code unit 0x005C (REVERSE SOLIDUS), *"x3"*, and the code unit whose numeric value is the numeric value of _c_.
+            1. If _escaped_ is the empty String, and _c_ is matched by |DecimalDigit| or |AsciiLetter|, then
+              1. NOTE: Escaping a leading digit ensures that output corresponds with pattern text which may be used after a `\0` character escape or a |DecimalEscape| such as `\1` and still match _S_ rather than be interpreted as an extension of the preceding escape sequence. Escaping a leading ASCII letter does the same for the context after `\c`.
+              1. Let _hex_ be Number::toString(𝔽(_c_), 16).
+              1. Set _escaped_ to the string-concatenation of the code unit 0x005C (REVERSE SOLIDUS), *"x"*, and StringPad(_hex_, 2, *"0"*, ~start~).
             1. Else,
               1. Set _escaped_ to the string-concatenation of _escaped_ and EncodeForRegExpEscape(_c_).
           1. Return _escaped_.