s = "hello"
s.concat(" world") #the same as <<
s.insert(5, " there") #s[5] = " there"
s.slice(0,5) #the same as s[0,5]
s.slice!(5,6) #same as s[5,6] = ""
s.eql?("hello world") #true

#the length of string
s = "hello"
s.length #5
s.bytesize #5 中文为每字3byte,ASCII为1byte
s.size # 5
s.empty? #false
"".empty? #true

s = "hello"
s.index('l') #2
s.index(?l) #2
s.index(/l+/) #2
s.index('l',3) #the first position after position 3 of l
s.index('Ruby') #nil
s.rindex('l') #3
s.rindex('l',2) #2  index of rightmost l at or before position 2

s.start_with? "hell" #true
s.end_with? "bells" #false

s.include?('ll') #true
s.include?(?H) #false

s =~ /[aeiou]{2}/ #nil
s.match(/[aeiou]/) {|m| m.to_s} #e return first

"this is it".split #this is it
"hello".split('l') #['he','','o']
"1, 2,3".split(/,\s*/) #1,2,3

"banana".partition("an") #["b", "an", "ana"]
"banana".rpartition("an") #["ban", "an", "a"]
"a123b".partition(/\d+/) #["a", "123", "b"]

s = "hello"
s.sub('l', 'L') #replace the first l to L
s.gsub("l", "L") #replace all the l to L
s.sub!(/(.)(.)/, '\2\1') #swap first character
s.sub!(/(.)(.)/, '\\2\\1') #"ehllo" Double backslashes for double quotes

"hello world".gsub(/\b./) { |match| match.upcase } #Hello World

s = "hello"
s.upcase #HELLO
s.downcase #hello
s.capitalize #Hello
s.swapcase #HELLO

s.casecmp("HELLO") #0 case insensitive comparasion

s = "hello\r\n"
s.chomp #"hello"
s.chomp("o") #hell
$/ = ";" #set global separator $/ to semicolon
"hello;".chomp #hello

s = "hello\n"
s.chop #hello  remove line terminator removed, redo will be hell

s = "\t hello \n"
s.strip #hello
s.lstrip #hello \n
s.rstrip #\t hello

s = "x"
s.ljust(3) #"x  "
s.rjust(3) #"  x"
s.center(3) #" x "
s.center(5, '-') #"--x--"
s.center(7, '-=') #"-=-x-=-"

s = "A\nB"
s.each_byte { |b| print b, " " } #65 10 66
s.each_line { |l| l.chomp } #AB

s.each_char { |c| print c, " " } #A \n B

0.upto(s.length - 1) {|n| print s[n,1], " "}
"a".succ #b
"aaz".next #aba
"a".upto("e") {|c| print c} #abcde

"hello".reverse #olleh
"hello\n".dump #escape special character "\"hello\\n\""
"hello\n".inspect #the same as dump

"hello".tr("aeiou", "AEIOU") #hEllO capitalize vowels
"head".tr_s("aeiou", "*") #h*d #remove duplicates

"hello".sum #weak 16-bit checksum
"hello".sum(8) #8 bit checksum
"hello".crypt("ab") #crypt hello with salt ab。"abl0JrMf6tlhw"

"hello".count('aeiou') #2
"hello".delete('aeiou') #hll
"hello".squeeze('a-z') #helo remove runs of letters.
"hello".count('a-z', '^aeiou') #3 str.count([other_str]+) Each other_str parameter defines a set of characters to count. The intersection of these sets defines the characters to count in str. Any other_str that starts with a caret (^) is negated. The sequence c1–c2 means all characters between c1 and c2.
"hello".delete('a-z', '^aeiou') #eo str.delete([other_str]+) Returns a copy of str with all characters in the intersection of its arguments deleted. Uses the same rules for building the set of characters as String#count.