Matching letter grades within a body of text - regex

Question

I am trying to write a regex expression for matching letter grades embedded within a string however, I am having some difficulty with certain characters. These characters are commas, backslashes, forward slashes, or apostrophes at word boundaries.
These strings may consist of either just a letter grade, or a mixture of a letter grade and notes left by an instructor. The valid range for these grades is anything from an A+ to D-, with an F reserved for failures. For a particular letter such as C the valid grades are: C+, C, or C-. Grades will never appear embedded within another word. Examples of some of these strings are as follows:
string1: "A+"
string2: "B. Submitted with deferral"
string3: "F. Could not read M/C answer sheet."
string4: "C+"
string5: "Received a B- with late submission penalty."
The expression that I have tried thus far is as follows:
(\b[A-D]\b[+-]?)|\bF\b)
For string1 and string2, this will produce the following matches
"A+"
"B. Submitted with deferral"
For string3 this expression should match
F. Could not read M/C answer sheet.
But instead matches
F. Could not read M/C answer sheet.
Any assistance would be much appreciated.
Edit:
For clarity a substring is a letter grade if and only if:
It is if the form A+, A, A-, B+, B, B-, ..., D+, D, D-, with F (without a sign) reserved for a failing grade
It is not embedded in a word, for example FOA+O would not match A+. Likewise substrings such as AC or FB should produce no matches
Letters separated by characters such as \ / ?' should not be matched, for example A/C, B+'C, F\D should not produce matches, whereas A, C or A,C should match both letters.
Letter separated by periods such as B.A. should not result in matches. Whereas an letter occurring at the end of a sentence such as A. may be considered a match.
Consider the following example strings
string1: "A-- A-C, A\D, F/A, D'C, A,C, B+D, C-C, AB, XA, B.A. C C,
Cat, F, C+, B-."
string2: " A "
string3: "B+."
string4: "X"
string5: "F"
in these strings the only valid matches should be
string1: "A-- A-C, A\D, F/A, D'C, A,C, B+D, C-C, AB, XA, B.A. C
C, Cat, F, C+, B-."
string2: " A "
string3: "B+."
string5: "F"

Answer 1

I'm not sure which regex engine you're using but the following regex works for all of the test cases you presented:
See regex in use here
(?<=^|[\s,])(?:[A-D][-+]?|F)(?=[-+.]\B|[\s,]|$)
(?<=^|[\s,]) Lookbehind ensuring what precedes is either of the following options:
^ Asserts position at the start of the line.
[\s,] Match any whitespace character or the comma character.
(?:[A-D][-+]?|F) Match either of the following options:
[A-D][-+]? Match the following:
[A-D] Match any character in the range from A to D in the ASCII table (ABCD).
[-+]? Optionally match any character in the set (- or +)
F Match this literally.
(?=[-+.]\B|[\s,]|$) Lookahead ensuring what proceeds is either of the following options:
[-+.]\B Matches any character in the set (-+.), followed by an assertion for anything that doesn't match a word boundary (ensures what follows is not a letter).
[\s,] Matches any whitespace character or the comma character.
$ Asserts position at the end of the line.
Alternatives
Fixed-width lookbehind - see in use here
(?:^|(?<=[\s,]))(?:[A-D][-+]?|F)(?=[-+.]\B|[\s,]|$)
Without lookbehind (uses capture group instead) - see in use here
(?:^|[\s,])([A-D][-+]?|F)(?=[-+.]\B|[\s,]|$)

Answer 2

The "C" in "M/C" is matched because \b considers the "/" a valid word boundary.
(?<=^|\s)[A-F][+-]{0,1}(?=\W)
This regular expression will match letter grades that are either at the beginning of the line (^), or are preceded with whitespace (\s). The positive lookbehind (?<=) ensures that the leading whitespace is not considered part of the match.
After the letter grade, we have (?=\W), which will require one non-word character, using positive lookahead to exclude the boundary character from the match.

Answer 3

Your original expression is just fine, yet this expression has a start anchor, that might be helping us here:
(?<=^|\s)\b[A-DF]\b[+-]?
Demo 1
Or with capturing group:
(?<=^|\s)(\b[A-DF]\b[+-]?)
Demo 2
Or without lookarounds, these might work:
(?:^|\s)(\b[A-DF]\b[+-]?)
(^|\s)(\b[A-DF]\b[+-]?)
^(\b[A-DF]\b[+-]?)|\s(\b[A-DF]\b[+-]?)