table-to-regex.sed

s/[[:space:]]*$//  # remove trailing whitespace

/^$\|^[[:space:]]*#/{p; b}  # empty and sed-style comment lines: print and branch
                            # printing keeps line numbers; for referencing errors

/^\([Fs]\)\(.\)\(.*\2\)\{4\}/{  # too many delims ERROR
      s/^/# error + # /p        # print a flagged/commented error
      b }                       # branch

/^\([Fs]\)\(.\)\(.*\2\)\{3\}/{                  # this may be a long-form 2nd delimiter
   /^\([Fs]\)\(.\)\(.*\2[[:space:]]*\2.*\2\)/{  # is long-form 2nd delimiter OK?
      s/^\([Fs]\)\(.\)\(.*\)\2[[:space:]]*\2\(.*\)\2\(.*\)/\1\2\n\3\n\4\n\5/
      t OK                                      # branch on true to :OK
   }; s/^/# error L # /p                        # print a flagged/commented error
      b }                                       # branch: long-form 2nd delimiter ERROR

/^\([Fs]\)\(.\)\(.*\2\)\{2\}/{     # this may be short-form delimiters
   /^\([Fs]\)\(.\)\(.*\2.*\2\)/{   # is short-form delimiters OK?
      s/^\([Fs]\)\(.\)\(.*\)\2\(.*\)\2\(.*\)/\1\2\n\3\n\4\n\5/
      t OK                         # branch on true to :OK
   }; s/^/# error S # /p           # print a flagged/commented error
      b }                          # branch: short-form delimiters ERROR

{ s/^/# error - # /p        # print a flagged/commented error
  b }                       # branch: too few delimiters ERROR

:OK     # delimiters are okay
#============================
h   # copy the pattern-space to the hold space

# NOTE: /^s/ lines are considered to contain regex patterns, not FIXED strings.
/^s/{    s/^s\(.\)\n/s\1/   # shrink long-form delimiter to short-form
     :s; s/^s\(.\)\([^\n]*\)\n/s\1\2\1/; t s  # branch on true to :s
      p; b }                                  # print and branch

# The following code handles FIXED-string /^F/ lines

s/^F.\n\([^\n]*\)\n.*/\1/  # isolate the literal find-string in the pattern-space
s/[]\/$*.^|[]/\\&/g        # convert the literal find-string into a regex of itself
H                          # append \n + find-regex to the hold-space

g   # Copy the modified hold-space back into the pattern-space

s/^F.\n[^\n]*\n\([^\n]*\)\n.*/\1/  # isolate the literal repl-string in the pattern-space
s/[\/&]/\\&/g                      # convert the literal repl-string into a regex of itself
H                                  # append \n + repl-regex to the hold-space

g   # Copy the modified hold-space back into the pattern-space

# Rearrange pattern-space into a / delimited command: s/find/repl/...
s/^\(F.\)\n\([^\n]*\)\n\([^\n]*\)\n\([^\n]*\)\n\([^\n]*\)\n\([^\n]*\)$/s\/\5\/\6\/\4/

p   # Print the modified find-and-replace regular expression line