19.8.28 record it flutter in a string of operations

print length of the string length. "IsEmpty" determines whether the string is empty.

"Contains () contains a string." "Substring (startIndex, endIndex) interception of a string." "

startsWith () "whether at the beginning of a string." endsWith () whether an end of the string. "

"IndexOf ()" subscript of a string. "LastIndexOf ()" final index of the first occurrence of a string. "

toLowerCase () "to lowercase," toUpperCase () "to uppercase.

"Trim ()" Clear a space. "TrimLeft ()" Clear front spaces. "TrimRight ()" clear space behind. "

split () "split the string." replace (), replaceAll ( 'old', 'new') "replacement string.