Make a note about ${s#prefix} ${s%suffix} and unicode.

core/libstr.py

@@ -191,13 +191,20 @@ def DoUnarySuffixOp(s, op, arg):

  # For patterns, do fnmatch() in a loop.
  #
  # TODO: Check another fast path first?
  # TODO: The loop needs to iterate over code points, not bytes!
  # - The forward case can probably be handled in a similar manner.
  # - The backward case might be handled by pre-calculating an array of start
  #   positions with _NextUtf8Char.
  #
  # TODO: Another potential fast path:
  #
  # v=aabbccdd
  # echo ${v#*b}  # strip shortest prefix
  #
  # If the whole thing doesn't match '*b*', then no test can succeed.  So we
  # can fail early.  Conversely echo ${v%%c*} and '*c*'.
  #
  # (Although honestly this whole construct is nuts and should be deprecated.)

  n = len(s)
  if op.op_id == Id.VOp1_Pound:  # shortest prefix

spec/var-op-strip.test.sh

@@ -60,8 +60,7 @@ echo ${v%[[:alpha:]]}
## N-I mksh stdout: abc

#### Strip unicode prefix
# NOTE: LANG is set to utf-8.  Problem: there is no way to represent the
# invalid character!  Instead of stdout-json, how about stdout-bytes?
# NOTE: LANG is set to utf-8.
v='μ-'
echo ${v#?}  # ? is a glob that stands for one character
## stdout: -