正则表达式只匹配行首的字符 [英] Regular expression to match characters at beginning of line only

问题描述

我正在尝试处理正则表达式.我有一个包含多个字段的大型机文件.我有一个平面文件解析器，它根据每行的前三个字母区分几种类型的记录.如何编写前三个字母为CTR"的正则表达式.

I am trying to work on regular expressions. I have a mainframe file which has several fields. I have a flat file parser which distinguishes several types of records based on the first three letters of every line. How do I write a regular expression where the first three letters are 'CTR'.

推荐答案

行的开头还是字符串的开头?

Beginning of line or beginning of string?

/^CTR.*$/

/ = 分隔符
^ = 字符串开始
CTR = 文字点击率
$ = 字符串结束
.* = 除换行符外的零个或多个任何字符

/ = delimiter
^ = start of string
CTR = literal CTR
$ = end of string
.* = zero or more of any character except newline

/^CTR.*$/m

/ = 分隔符
^ = 行首
CTR = 文字点击率
$ = 行尾
.* = 除换行符之外的零个或多个任何字符
m = 启用多行模式，这将正则表达式设置为将每一行视为一个字符串，因此 ^ 和 $ 将匹配开始和结束线

/ = delimiter
^ = start of line
CTR = literal CTR
$ = end of line
.* = zero or more of any character except newline
m = enables multi-line mode, this sets regex to treat every line as a string, so ^ and $ will match start and end of line

在多行模式下，您仍然可以使用 \A\Z 永久锚点来匹配字符串的开头和结尾

While in multi-line mode you can still match the start and end of the string with \A\Z permanent anchors

/\ACTR.*\Z/m

\A = 表示字符串的开始
CTR = 文字点击率
.* = 除换行符之外的零个或多个任何字符
\Z = 字符串结束
m = 启用多行模式

\A = means start of string
CTR = literal CTR
.* = zero or more of any character except newline
\Z = end of string
m = enables multi-line mode

因此，另一种匹配行首的方法是这样的:

As such, another way to match the start of the line would be like this:

/(\A|\r|\n|\r\n)CTR.*/

或

/(^|\r|\n|\r\n)CTR.*/

\r = 回车/旧的 Mac OS 换行符
\n = 换行符/Unix/Mac OS X 换行符
\r\n = windows 换行符

\r = carriage return / old Mac OS newline
\n = line-feed / Unix/Mac OS X newline
\r\n = windows newline

注意，如果你打算在一些支持转义的程序字符串中使用反斜杠 \，比如 php 双引号 "" 那么你需要对它们进行转义先

Note, if you are going to use the backslash \ in some program string that supports escaping, like the php double quotation marks "" then you need to escape them first

所以要运行 \r\nCTR.* 你会使用它作为 "\\r\\nCTR.*"

so to run \r\nCTR.* you would use it as "\\r\\nCTR.*"

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