带尖括号的正则表达式行为 [英] Regex behaviour with angle brackets
问题描述
请向我解释为什么以下表达式不输出任何内容:
Please explain to me why the following expression doesn't output anything:
echo "<firstname.lastname@domain.com>" | egrep "<lastname@domain.com>"
但以下内容确实存在:
echo "<firstname.lastname@domain.com>" | egrep "\<lastname@domain.com>"
第一个的行为与预期的一样，但第二个的行为不应输出.是"\<"在正则表达式中被忽略还是引起其他特殊行为?
The behaviour of the first is as expected but the second should not output. Is the "\<" being ignored within the regex or causing some other special behaviour?
推荐答案
AS @hwnd说\<与单词的开头匹配. ie单词边界\b必须存在于起始单词字符之前(输入中\<之后的字符必须是单词字符)
AS @hwnd said \< matches the begining of the word. ie a word boundary \b must exists before the starting word character(character after \< in the input must be a word character),
在您的示例中，
echo "<firstname.lastname@domain.com>" | egrep "<lastname@domain.com>"
在上面的示例中，egrep检查姓氏字符串之前的文字<字符.但是没有，所以它什么也不打印.
In the above example, egrep checks for a literal < character present before the lastname string. But there isn't, so it prints nothing.
$ echo "<firstname.lastname@domain.com>" | egrep "\<lastname@domain.com>"
<firstname.**lastname@domain.com>**
但是在此示例中，在lastname字符串之前存在单词边界\b，因此它会打印匹配的字符.
But in this example, a word boundary \b exists before lastname string so it prints the matched characters.
更多示例:
$ echo "namelastname@domain.com" | egrep "\<e@domain.com"
$ echo "namelastname@domain.com" | egrep "\<lastname@domain.com"
$ echo "namelastname@domain.com" | egrep "\<com"
namelastname@domain.**com**
$ echo "<firstname.lastname@domain.com>" | egrep "\<@domain.com>"
$ echo "n-ame-lastname@domain.com" | egrep "\<ame-lastname@domain.com"
n-**ame-lastname@domain.com**
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