Here is the table properly converted to Markdown.
|Regex String|Description|Use Case|Explanation|
|:----|:----|:----|:----|
|^abc|Matches any string that starts with "abc"|To find lines in a text file that start with "abc"|^ asserts the start of a line, and abc is the literal string|
|abc$|Matches any string that ends with "abc"|To find lines in a text file that end with "abc"|abc is the literal string, and $ asserts the end of a line|
|a.b|Matches any string containing "a", any character, and then "b"|To find occurrences of "a" followed by any character and then "b"|a and b are literal characters, and . matches any character except a newline|
|\d{2,4}|Matches a sequence of 2 to 4 digits|To validate a year input with 2 to 4 digits|\d matches any digit, and {2,4} specifies a range of 2 to 4 occurrences|
|[A-Za-z]|Matches any single uppercase or lowercase letter|To find single letters in a text file|[A-Za-z] defines a character class containing all uppercase and lowercase letters|
|[^0-9]|Matches any character that is not a digit|To find non-digit characters in a text file|[^0-9] defines a character class with a negation (^) and a range for digits (0-9)|
|(abc|def)|Matches either "abc" or "def"|To find occurrences of either "abc" or "def" in a text file|
|a{3}|Matches exactly three consecutive "a" characters|To find occurrences of "aaa" in a text file|a is the literal character, and {3} specifies exactly 3 occurrences|
|\bword\b|Matches the word "word" as a whole word|To search for the whole word "word" in a text file|\b asserts a word boundary, and word is the literal string|
|(?i)abc|Matches "abc" case-insensitively|To find occurrences of "abc" regardless of case|(?i) sets the case-insensitive flag, and abc is the literal string|
|(\d+)\s+|Matches one or more digits followed by one or more whitespace characters|To find and separate numbers followed by spaces in a text file|(\d+) captures one or more digits, and \s+ matches one or more whitespace characters|
|(?<=@)\w+|Matches a word that follows the "@" symbol|To extract usernames from email addresses|(?<=@) is a positive lookbehind for the "@" symbol, and \w+ matches one or more word characters|
spam fails_task lang_mismatch pii not_appropriate hate_speech sexual_content quality toxicity humor helpfulness creativity violence