问题

I would like to match either "direction" (">" or "<" ) from a string like "->", "<==", "...". When the string contains no direction, I want to match "". More precisely, the equivalent Python expression would be:

">" if ">" in s else ("<" if "<" in s else "")

I first came up with this simple regular expression:

re.search(r"[<>]|\"", s)[0]

... but it evaluates to "" as soon as the direction is not in the first position. How would you do that?

">" if ">" in s else ("<" if "<" in s else "")

re.search(r"[<>]|", s)[0]

答案1

得分: 4

"A simpler version of Andrej Kesely's solution:

re.search('<|>|$', s)[0]
```"
<details>
<summary>英文:</summary>
A simpler version of [Andrej Kesely](https://stackoverflow.com/users/10035985)&#39;s solution:
```python
re.search(&#39;&lt;|&gt;|$&#39;, s)[0]

答案2

得分: 3

Here's the translated content without the code:

如果你想要一个纯粹的 re 解决方案（regex101）：

打印：

这将匹配第一个 < 或 > 字符串或字符串的末尾 $（然后返回空字符串 ''）。

编辑：简化了正则表达式，感谢 @InSync

import re
test_cases = [
    "->", "<==", "..."
]
for t in test_cases:
    print(re.search('(?=[<>]).|$', t)[0])

>
<

答案3

得分: 2

The reason this regex [<>]| returns "" if the first character is not < or > is because the alternation in this case always matches the position, returning a zero length match for re.search

Instead of writing if/else statements or a regex, you could also use next() which can return a default value and list the strings that you want to match in an array:

test_str = "this is a test string with > and <"
items = ["<", ">"]
item = next((i for i in items if i in test_str), "")
print(item)

test_str = "this is a test string with > and <"
items = ["<", ">"]
item = next((i for i in items if i in test_str), "")
print(item)