搜索關鍵字字符串NSSCanner:scanString（）用來查找某段文本中某個關鍵字出現的位置。
NSScanner類用于在字符串中掃描指定的字符，尤其是把它們翻譯/轉換為數字和別的字符串。可以在創建NSScaner時指定它的string屬性，然后scanner會按照你的要求從頭到尾地掃描這個字符串的每個字符。
scanner默認情況下會忽略空白字符和換行符。注意：對于忽略字符，大小寫是敏感的。

看下面的代碼：

(NSString*)extractBodyFromMessage:(NSString* )msg{NSString* body=msg;NSString* keyString=@"【來自網易郵箱的超大附件】";NSScanner*scanner=[NSScanner scannerWithString:body];[scannersetCaseSensitive:NO];BOOL b;while (![scanner isAtEnd]){b=[scannerscanString:keyString intoString:NULL];if(b) {body=[body substringToIndex:[scanner scanLocation]-keyString.length];break;}else{scanner.scanLocation++;}}return body;}

在while循環里面，我們首先從0位置開始掃描指定關鍵字，如果未找到，移動scanLocation位置，繼續從下一字符查找直至文本末尾；如果找到，直接返回子字符串。

scanUpToString在找到指定文本（keyString）后，會將scanLocation停止在keyString之前（scanString方法會將scanLocation移到keyString之后位置），這樣我們的substringToIndex:方法就不必再用scanLocation減去keyString的長度了：
body=[bodysubstringToIndex:[scanner scanLocation]];

其中，如果scanUpToString在文本的開頭（第一個字符）就找到目標文本，會返回NO，stringValue不會改變。

如果目標文本在receiver文本中被找到，返回YES并將scanLocation設置到已找到的目標文本之前。

如果目標文本未找到，從本次查找開始剩下的整個receiver文本被放入stringValue，scanLocation移動到源串的末尾，返回YES。
*
scanUpToString和scanString的區別*，
如果目標文本一開頭就出現目標文本（scanner默認是忽略空格和換行的，因此如果開頭有這兩個空白字符，scanner視若未見），二者將得到完全不同的結果：scanString會返回YES，而scanUpToString返回NO。
下面是scanstring方法的幾個示例：

# 需要導入模塊: from simplejson import _speedups [as 別名] # 或者: from simplejson._speedups import scanstring [as 別名] def _import_c_scanstring():try:from simplejson._speedups import scanstringreturn scanstringexcept ImportError:return None # 需要導入模塊: from simplejson import _speedups [as 別名] # 或者: from simplejson._speedups import scanstring [as 別名] def _import_c_scanstring():try:raise ImportError # because assumes simplejson in pathfrom simplejson._speedups import scanstringreturn scanstringexcept ImportError:return None # 需要導入模塊: from simplejson import _speedups [as 別名] # 或者: from simplejson._speedups import scanstring [as 別名] def __init__(self, encoding=None, object_hook=None, parse_float=None,parse_int=None, parse_constant=None, strict=True,object_pairs_hook=None):"""*encoding* determines the encoding used to interpret any:class:`str` objects decoded by this instance (``'utf-8'`` bydefault). It has no effect when decoding :class:`unicode` objects.Note that currently only encodings that are a superset of ASCII work,strings of other encodings should be passed in as :class:`unicode`.*object_hook*, if specified, will be called with the result of everyJSON object decoded and its return value will be used in place of thegiven :class:`dict`. This can be used to provide customdeserializations (e.g. to support JSON-RPC class hinting).*object_pairs_hook* is an optional function that will be called withthe result of any object literal decode with an ordered list of pairs.The return value of *object_pairs_hook* will be used instead of the:class:`dict`. This feature can be used to implement custom decodersthat rely on the order that the key and value pairs are decoded (forexample, :func:`collections.OrderedDict` will remember the order ofinsertion). If *object_hook* is also defined, the *object_pairs_hook*takes priority.*parse_float*, if specified, will be called with the string of everyJSON float to be decoded. By default, this is equivalent to``float(num_str)``. This can be used to use another datatype or parserfor JSON floats (e.g. :class:`decimal.Decimal`).*parse_int*, if specified, will be called with the string of everyJSON int to be decoded. By default, this is equivalent to``int(num_str)``. This can be used to use another datatype or parserfor JSON integers (e.g. :class:`float`).*parse_constant*, if specified, will be called with one of thefollowing strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``. Thiscan be used to raise an exception if invalid JSON numbers areencountered.*strict* controls the parser's behavior when it encounters aninvalid control character in a string. The default setting of``True`` means that unescaped control characters are parse errors, if``False`` then control characters will be allowed in strings."""self.encoding = encodingself.object_hook = object_hookself.object_pairs_hook = object_pairs_hookself.parse_float = parse_float or floatself.parse_int = parse_int or intself.parse_constant = parse_constant or _CONSTANTS.__getitem__self.strict = strictself.parse_object = JSONObjectself.parse_array = JSONArrayself.parse_string = scanstring self.memo = {}self.scan_once = make_scanner(self)

參考：
博客1地址
參考2地址
參考3地址

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