Python re.sub() 行首锚定 [英] Python re.sub() beginning-of-line anchoring

问题描述

考虑以下多行字符串:

<代码>>>印刷我可以把你比作夏日吗?你是更可爱，更温和的狂风确实撼动了五月的萌芽，夏天的租约太短了.

re.sub() 将所有出现的 and 替换为 AND:

<预><代码>>>>打印 re.sub("and", "AND", s)我可以把你比作夏日吗?你是更可爱，更温和的狂风确实撼动了五月的萌芽，而且夏天的租约日期太短了.

但是 re.sub() 不允许 ^ 锚定到行首，所以添加它不会导致 and> 被替换:

<预><代码>>>>打印 re.sub("^and", "AND", s)我可以把你比作夏日吗?你是更可爱，更温和的狂风确实撼动了五月的萌芽，夏天的租约太短了.

如何将 re.sub() 与行首 (^) 或行尾 ($) 锚点?

解决方案

您忘记启用多行模式.

re.sub("^and", "AND", s, flags=re.M)

<块引用>

re.M
re.MULTILINE

指定时，模式字符 '^' 匹配字符串的开头和每行的开头(紧跟在每个换行符之后)；并且模式字符 '$' 在字符串的末尾和每行的末尾(紧接在每个换行符之前)匹配.默认情况下，'^' 只匹配字符串的开头，'$' 只匹配字符串的结尾和换行符(如果有)之前的位置字符串的结尾.

来源

flags 参数不适用于 2.7 以上的 Python；所以在这些情况下，你可以直接在正则表达式中设置它，如下所示:

re.sub("(?m)^and", "AND", s)

Consider the following multiline string:

>> print s
shall i compare thee to a summer's day?
thou art more lovely and more temperate
rough winds do shake the darling buds of may,
and summer's lease hath all too short a date.

re.sub() replaces all the occurrence of and with AND:

>>> print re.sub("and", "AND", s)
shall i compare thee to a summer's day?
thou art more lovely AND more temperate
rough winds do shake the darling buds of may,
AND summer's lease hath all too short a date.

But re.sub() doesn't allow ^ anchoring to the beginning of the line, so adding it causes no occurrence of and to be replaced:

>>> print re.sub("^and", "AND", s)
shall i compare thee to a summer's day?
thou art more lovely and more temperate
rough winds do shake the darling buds of may,
and summer's lease hath all too short a date.

How can I use re.sub() with start-of-line (^) or end-of-line ($) anchors?

解决方案

You forgot to enable multiline mode.

re.sub("^and", "AND", s, flags=re.M)

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.

source

The flags argument isn't available for python older than 2.7; so in those cases you can set it directly in the regular expression like so:

re.sub("(?m)^and", "AND", s)

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