如何用正则表达式匹配并替换单词的单复数形式

先回顾基础的替换逻辑：

假设我们有这段语句：

sentence = "A cow runs on the grass"

要把单词cow替换为<SPECIAL>标记，可执行以下代码：

import re
to_replace = "cow"
# 替换后结果：A <SPECIAL> runs on the grass
sentence = re.sub(rf"(?!\B\w)({re.escape(to_replace)})(?<!\w\B)", "<SPECIAL>", sentence, count=1)

如果要同时支持替换加s的复数形式，只需在正则里添加s?：

sentence = "The cows run on the grass"
to_replace = "cow"
# 替换后结果：The <SPECIAL> run on the grass
sentence = re.sub(rf"(?!\B\w)({re.escape(to_replace) + 's?'})(?<!\w\B)", "<SPECIAL>", sentence, count=1)

这段代码能同时匹配单数cow和复数cows。

现在要实现更通用的规则：同时匹配单词的单数形式、以s结尾的复数、以es结尾的复数（忽略其他复杂复数规则），适配以下测试场景：

to_replace = "cow"
sentence1 = "The cow runs on the grass"
sentence2 = "The cows run on the grass"
# --------------
to_replace = "gas"
sentence3 = "There are many natural gases"

解决方案 #

核心是在正则中添加分支化的可选后缀，优先匹配es再匹配s，确保长后缀不会被拆分。完整代码如下：

import re

# 测试cow的单数/复数场景
to_replace = "cow"
sentence1 = "The cow runs on the grass"
sentence2 = "The cows run on the grass"

# 构建通用匹配正则
pattern = rf"(?!\B\w)({re.escape(to_replace)})(?:es|s)?(?<!\w\B)"

# 替换并输出结果
result1 = re.sub(pattern, "<SPECIAL>", sentence1, count=1)
print(result1)  # 输出：The <SPECIAL> runs on the grass

result2 = re.sub(pattern, "<SPECIAL>", sentence2, count=1)
print(result2)  # 输出：The <SPECIAL> run on the grass

# 测试gas的复数场景
to_replace = "gas"
sentence3 = "There are many natural gases"
result3 = re.sub(pattern, "<SPECIAL>", sentence3, count=1)
print(result3)  # 输出：There are many natural <SPECIAL>

正则说明 #

• (?:es|s)?：非捕获组，?:避免捕获后缀内容，|实现分支选择（先匹配es再匹配s），?表示后缀可选（对应单数形式）
• re.escape(to_replace)：自动转义目标单词中的正则特殊字符（如.、*等），避免正则语法错误
• (?!\B\w)和(?<!\w\B)：确保匹配完整单词，不会误替换单词中的子串（比如不会替换coward里的cow）

内容的提问来源于stack exchange，提问作者gorjan