pandas 替换和不区分大小写 [英] Pandas .str.replace and case insensitivity

问题描述

在以下示例中，使替换大小写不敏感似乎没有任何效果(我想用 jr替换 jr.或 Jr. ):

In [0]: pd.Series('Jr. eng').str.replace('jr.', 'jr', regex=False, case=False)
Out[0]: 0    Jr. eng

为什么?我有什么误会?

解决方案

case参数实际上是一种方便的方法，可以代替指定flags=re.IGNORECASE.如果替换不是基于正则表达式的，则与替换无关.

因此，当regex=True时，这些是您可能的选择:

pd.Series('Jr. eng').str.replace(r'jr\.', 'jr', regex=True, case=False)
# pd.Series('Jr. eng').str.replace(r'jr\.', 'jr', case=False)

0    jr eng
dtype: object

或者，

pd.Series('Jr. eng').str.replace(r'jr\.', 'jr', regex=True, flags=re.IGNORECASE)
# pd.Series('Jr. eng').str.replace(r'jr\.', 'jr', flags=re.IGNORECASE)

0    jr eng
dtype: object

通过将不区分大小写的标志作为?i的一部分纳入模式，您还可以变得轻松自如并绕过两个关键字参数.见

pd.Series('Jr. eng').str.replace(r'(?i)jr\.', 'jr')
0    jr eng
dtype: object

注意
您需要在正则表达式模式下转义句点\.，因为 未转义的点是具有不同含义的元字符(匹配 任何字符).如果您想动态地转义模式中的元字符，可以使用 re.escape .

有关标志和锚点的更多信息，请参见本节docs 和 re HOWTO . /p>

---

来自来源代码，很明显，如果regex=False，则忽略"case"参数.见

# Check whether repl is valid (GH 13438, GH 15055)
if not (is_string_like(repl) or callable(repl)):
    raise TypeError("repl must be a string or callable")

is_compiled_re = is_re(pat)
if regex:
    if is_compiled_re:
        if (case is not None) or (flags != 0):
            raise ValueError("case and flags cannot be set"
                             " when pat is a compiled regex")
    else:
        # not a compiled regex
        # set default case
        if case is None:
            case = True

        # add case flag, if provided
        if case is False:
            flags |= re.IGNORECASE
    if is_compiled_re or len(pat) > 1 or flags or callable(repl):
        n = n if n >= 0 else 0
        compiled = re.compile(pat, flags=flags)
        f = lambda x: compiled.sub(repl=repl, string=x, count=n)
    else:
        f = lambda x: x.replace(pat, repl, n)

您可以看到case参数仅在if语句内被选中.

IOW，唯一的方法是确保regex=True以便替换基于正则表达式.

Making the replace case insensitive does not seem to have an effect in the following example (I want to replace jr. or Jr. with jr):

In [0]: pd.Series('Jr. eng').str.replace('jr.', 'jr', regex=False, case=False)
Out[0]: 0    Jr. eng

Why? What am I misunderstanding?

解决方案

The case argument is actually a convenience as an alternative to specifying flags=re.IGNORECASE. It has no bearing on replacement if the replacement is not regex-based.

So, when regex=True, these are your possible choices:

pd.Series('Jr. eng').str.replace(r'jr\.', 'jr', regex=True, case=False)
# pd.Series('Jr. eng').str.replace(r'jr\.', 'jr', case=False)

0    jr eng
dtype: object

Or,

pd.Series('Jr. eng').str.replace(r'jr\.', 'jr', regex=True, flags=re.IGNORECASE)
# pd.Series('Jr. eng').str.replace(r'jr\.', 'jr', flags=re.IGNORECASE)

0    jr eng
dtype: object

You can also get cheeky and bypass both keyword arguments by incorporating the case insensitivity flag as part of the pattern as ?i. See

pd.Series('Jr. eng').str.replace(r'(?i)jr\.', 'jr')
0    jr eng
dtype: object

Note
You will need to escape the period \. in regex mode, because the unescaped dot is a meta-character with a different meaning (match any character). If you want to dynamically escape meta-chars in patterns, you can use re.escape.

For more information on flags and anchors, see this section of the docs and re HOWTO.

---

From the source code, it is clear that the "case" argument is ignored if regex=False. See

# Check whether repl is valid (GH 13438, GH 15055)
if not (is_string_like(repl) or callable(repl)):
    raise TypeError("repl must be a string or callable")

is_compiled_re = is_re(pat)
if regex:
    if is_compiled_re:
        if (case is not None) or (flags != 0):
            raise ValueError("case and flags cannot be set"
                             " when pat is a compiled regex")
    else:
        # not a compiled regex
        # set default case
        if case is None:
            case = True

        # add case flag, if provided
        if case is False:
            flags |= re.IGNORECASE
    if is_compiled_re or len(pat) > 1 or flags or callable(repl):
        n = n if n >= 0 else 0
        compiled = re.compile(pat, flags=flags)
        f = lambda x: compiled.sub(repl=repl, string=x, count=n)
    else:
        f = lambda x: x.replace(pat, repl, n)

You can see the case argument is only checked inside the if statement.

IOW, the only way is to ensure regex=True so that replacement is regex-based.

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