最近寫了個python抓取必應搜索首頁http://cn.bing.com/的背景圖片并將此圖片更換為我的電腦桌面的程序，在正則匹配圖片url時遇到了匹配失敗問題。

要抓取的圖片地址如圖所示：

首先，使用這個pattern

reg = re.compile('.*g_img={url: "(http.*?jpg)"')

無論怎么匹配都匹配不到，后來把網頁源碼抓下來放在notepad++中查看，并用notepad++的正則匹配查找，很輕易就匹配到了，如圖：

后來我寫了個測試代碼，把圖片地址在的那一行保存在一個字符串中，很快就匹配到了，如下面代碼所示，data是匹配不到的，然而line是可以匹配到的。

# -*-coding:utf-8-*-import osimport re f = open('bing.html','r') line = r'''Bnp.Internal.Close(0,0,60056); } });;g_img={url: "https://az12410.vo.msecnd.net/homepage/app/2016hw/BingHalloween_BkgImg.jpg",id:'bgDiv',d:'200',cN'''data = f.read().decode('utf-8','ignore').encode('gbk','ignore') print " " reg = re.compile('.*g_img={url: "(http.*?jpg)"') if re.match(reg, data):  m1 = reg.findall(data)  print m1[0]else:  print("data Not match .")  print 20*'-'#print lineif re.match(reg, line):  m2 = reg.findall(line)  print m2[0]else:  print("line Not match .")

由此可見line和data是有區別的，什么區別呢？那就是data是多行的，包含換行符，而line是單行的，沒有換行符。我有在字符串line中加了換行符，結果line沒有匹配到。

到這了原因就清楚了。原因就在這句話

re.compile('.*g_img={url: "(http.*?jpg)"')。

后來翻閱python文檔，發現re.compile()這個函數的第二個可選參數flags。這個參數是re中定義的常量，有如下常量

re.DEBUG Display debug information about compiled expression.re.I re.IGNORECASE Perform case-insensitive matching; expressions like [A-Z] will match lowercase letters, too. This is not affected by the current locale.

re.L re.LOCALE Make /w, /W, /b, /B, /s and /S dependent on the current locale.

re.M re.MULTILINE When specified, the pattern character '^' matches at the beginning of the string and at the beginning of each line (immediately following each newline); and the pattern character '$' matches at the end of the string and at the end of each line (immediately preceding each newline). By default, '^' matches only at the beginning of the string, and '$' only at the end of the string and immediately before the newline (if any) at the end of the string.

re.S re.DOTALL Make the '.' special character match any character at all, including a newline; without this flag, '.' will match anything except a newline.re.U re.UNICODE Make /w, /W, /b, /B, /d, /D, /s and /S dependent on the Unicode character properties database.New in version 2.0.