Python:替换字符串，从列表中匹配

Question

尝试匹配和标记基于字符的 n-gram。字符串

txt = "how does this work"

要与列表中的 n-gram 匹配

ngrams = ["ow ", "his", "s w"]

并标有<> – 但是，前提是没有前面的开价。我正在寻找此字符串的输出是 h<ow >does t<his w>ork (注意 2^-nd 部分中的双重匹配，但仅在一对预期引号内)。

然而，我为此尝试过的 for 循环根本没有产生想要的输出:

switch = False

for i in txt:
    if i in "".join(ngrams) and switch == False:
        txt = txt.replace(i, "<" + i)
        switch = True
    if i not in "".join(ngrams) and switch == True:
        txt = txt.replace(i, ">" + i)
        switch = False

print(txt)

如有任何帮助，我们将不胜感激。

Answer 1

此解决方案使用 str.find 方法在 txt 字符串中查找 ngram 的所有副本，将每个副本的索引保存到 indices 设置以便我们可以轻松处理重叠匹配。

然后我们复制 txt，一个字符一个字符地复制到 result 列表，在需要的地方插入尖括号。此策略比使用多个 .replace 调用插入尖括号更有效，因为每个 .replace 调用都需要重建整个字符串。

我稍微扩展了您的数据以说明我的代码处理 ngram 的多个副本。

txt = "how does this work now chisolm"
ngrams = ["ow ", "his", "s w"]
print(txt)
print(ngrams)

# Search for all copies of each ngram in txt
# saving the indices where the ngrams occur
indices = set()
for s in ngrams:
    slen = len(s)
    lo = 0
    while True:
        i = txt.find(s, lo)
        if i == -1:
            break
        lo = i + slen 
        print(s, i)
        indices.update(range(i, lo-1))

print(indices)

# Copy the txt to result, inserting angle brackets
# to show matches
switch = True
result = []
for i, u in enumerate(txt):
    if switch:
        if i in indices:
            result.append('<')
            switch = False
        result.append(u)
    else:
        result.append(u)
        if i not in indices:
            result.append('>')
            switch = True

print(''.join(result))

输出

how does this work now chisolm
['ow ', 'his', 's w']
ow  1
ow  20
his 10
his 24
s w 12
{1, 2, 10, 11, 12, 13, 20, 21, 24, 25}
h<ow >does t<his w>ork n<ow >c<his>olm

---

如果你想合并相邻的组，我们可以使用 str.replace 方法轻松地做到这一点。但要使其正常工作，我们需要预处理原始数据，将所有空格转换为单个空格。一种简单的方法是拆分数据并重新加入。

txt = "how  does this\nwork  now chisolm hisow"
ngrams = ["ow", "his", "work"]

#Convert all whitespace to single spaces
txt = ' '.join(txt.split())

print(txt)
print(ngrams)

# Search for all copies of each ngram in txt
# saving the indices where the ngrams occur
indices = set()
for s in ngrams:
    slen = len(s)
    lo = 0
    while True:
        i = txt.find(s, lo)
        if i == -1:
            break
        lo = i + slen 
        print(s, i)
        indices.update(range(i, lo-1))

print(indices)

# Copy the txt to result, inserting angle brackets
# to show matches
switch = True
result = []
for i, u in enumerate(txt):
    if switch:
        if i in indices:
            result.append('<')
            switch = False
        result.append(u)
    else:
        result.append(u)
        if i not in indices:
            result.append('>')
            switch = True

# Convert the list to a single string
output = ''.join(result)

# Merge adjacent groups
output = output.replace('> <', ' ').replace('><', '')
print(output)

输出

how does this work now chisolm hisow
['ow', 'his', 'work']
ow 1
ow 20
ow 34
his 10
his 24
his 31
work 14
{32, 1, 34, 10, 11, 14, 15, 16, 20, 24, 25, 31}
h<ow> does t<his work> n<ow> c<his>olm <hisow>