不必使用正则表达式(对于示例字符串)

>>> s
'this is a paragraph with<[1> in between</[1> and then there are cases ... where the<[99> number ranges from 1-100</[99>. \nand there are many other lines in the txt files\nwith<[3> such tags </[3>\n'

>>> for w in s.split(">"):
...   if "<" in w:
...      print w.split("<")[0]
...
this is a paragraph with
 in between
 and then there are cases ... where the
 number ranges from 1-100
.
and there are many other lines in the txt files
with
 such tags

我会像这样(在评论中解释了regex):

import re

# If you need to use the regex more than once it is suggested to compile it.
pattern = re.compile(r"</{0,}\[\d+>")

# <\/{0,}\[\d+>
# 
# Match the character “<” literally «<»
# Match the character “/” literally «\/{0,}»
#    Between zero and unlimited times, as many times as possible, giving back as needed (greedy) «{0,}»
# Match the character “[” literally «\[»
# Match a single digit 0..9 «\d+»
#    Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+»
# Match the character “>” literally «>»

subject = """this is a paragraph with<[1> in between</[1> and then there are cases ... where the<[99> number ranges from 1-100</[99>. 
and there are many other lines in the txt files
with<[3> such tags </[3>"""

result = pattern.sub("", subject)

print(result)

如果你想了解更多关于正则表达式的知识，我推荐阅读Jan Goyvaerts和Steven Levithan写的正则表达式烹饪书。

这个测试片段应该做到这一点:

import re
line = re.sub(r"</?\[\d+>", "", line)

编辑:下面是一个注释版本，解释它是如何工作的:

line = re.sub(r"""
  (?x) # Use free-spacing mode.
  <    # Match a literal '<'
  /?   # Optionally match a '/'
  \[   # Match a literal '['
  \d+  # Match one or more digits
  >    # Match a literal '>'
  """, "", line)

regex很有趣!但我强烈建议花一两个小时学习基础知识。对于初学者，你需要了解哪些字符是特殊的:需要转义的“元字符”(即在前面放一个反斜杠——字符类内外的规则是不同的)。有一个很好的在线教程:www.regular-expressions.info。你在那里花费的时间将会为你带来数倍的回报。regex快乐!

import os, sys, re, glob

pattern = re.compile(r"\<\[\d\>")
replacementStringMatchesPattern = "<[1>"

for infile in glob.glob(os.path.join(os.getcwd(), '*.txt')):
   for line in reader: 
      retline =  pattern.sub(replacementStringMatchesPattern, "", line)         
      sys.stdout.write(retline)
      print (retline)

不必使用正则表达式(对于示例字符串)

>>> s
'this is a paragraph with<[1> in between</[1> and then there are cases ... where the<[99> number ranges from 1-100</[99>. \nand there are many other lines in the txt files\nwith<[3> such tags </[3>\n'

>>> for w in s.split(">"):
...   if "<" in w:
...      print w.split("<")[0]
...
this is a paragraph with
 in between
 and then there are cases ... where the
 number ranges from 1-100
.
and there are many other lines in the txt files
with
 such tags

字符串对象的Replace方法不接受正则表达式，只接受固定字符串(参见文档:http://docs.python.org/2/library/stdtypes.html#str.replace)。

你必须使用re模块:

import re
newline= re.sub("<\/?\[[0-9]+>", "", line)