"<a>foo</a>".between("<a>", "</a>") // "foo"
"<a><a>foo</a></a>".between("<a>", "</a>") // "<a>foo</a>"
"<a>foo".between("<a>", "</a>") // nil
"Some strings } are very {weird}, dont you think?".between("{", "}") // "weird"
"<a></a>".between("<a>", "</a>") // nil
"<a>foo</a>".between("<a>", "<a>") // nil

let matches = "Swift Moji 9876".matches(regex: "^(.+)\\s(\\d{4})")

// first is the entire capture
matches.first // "Swift Moji 9876"

// individual capture groups begin from [1]
matches[1] // "Swift Moji"
matches.last // "2017"

"abc".first() // "a"

"abc".second() // "b"

"abc".last() // "c"

String.hiragana()　
//ぁあぃいぅうぇえぉおかがきぎくぐけげこごさざしじすずせぜそぞただちぢっつづてでとどなにぬねのはばぱひびぴふぶぷへべぺほぼぽまみむめもゃやゅゆょよらりるれろゎわゐゑをんゔゕゖ

String.katakana()　
//ァアィイゥウェエォオカガキギクグケゲコゴサザシジスズセゼソゾタダチヂッツヅテデトドナニヌネノハバパヒビピフブプヘベペホボポマミムメモャヤュユョヨラリルレロヮワヰヱヲンヴヵヶ

"abc_def".camelized() // "AbcDef"

"UserId".snakecased() // "user_id"

"   abcdef   ".trimmed()      // "abcdef"
"   abcdef   ".trimmedLeft()  // "abcdef   "
"   abcdef   ".trimmedRight() // "   abcdef"

"abc".uppercasedFirst() // "Abc"

"ABC".lowercasedFirst() // "aBC"

"Abc".isCapitalized() // "true"
"abc".isCapitalized() // "false"

"ABC".isOnlyUppercased()   // "true"
"A_B_C".isOnlyUppercased() // "false"

"abc".isOnlyLowercased()   // "true"
"a_b_c".isOnlyLowercased() // "false"

"UserId".isUpperCamelized() // "true"
"userId".isUpperCamelized() // "false"

"userId".isLowerCamelized() // "true"
"UserId".isLowerCamelized() // "false"

"abc".isAlphabet() // "true"
"123".isAlphabet() // "false"
"   ".isAlphabet() // "false"

"-1.983388".isNumeric() // "true"
"abc".isNumeric()       // "false"

"あいうえお".isHiragana() // "true"
"abc".isHiragana()      // "false"

"アイウエオ".isKatakana() // "true"
"abc".isKatakana()      // "false"