"{4} matches the preceding item up to four (or any number you enter in the braces) times"
It matches exactly 4 times. "a{4}" will match "aaaa", but not "aaa"
"grep [A,C].+a example.txt"
This one's not wrong, per se, but misleading. It implies that if you want to look for "A" or "C" that [A,C] is what you want. That's looking for "A" or "," or "C".
Good article, but some minor nits:
"{4} matches the preceding item up to four (or any number you enter in the braces) times"
It matches exactly 4 times. "a{4}" will match "aaaa", but not "aaa"
"grep [A,C].+a example.txt"
This one's not wrong, per se, but misleading. It implies that if you want to look for "A" or "C" that [A,C] is what you want. That's looking for "A" or "," or "C".