In Python3 I have 3 strings:

string1 = '[Foo] bar1'
string2 = '[foo] bar2'
string3 = 'bar3'

I would like to add prefix "Foo" for each string, except first string that already have it. And replace lower "f" to upper "F" in string2. So, I mean, that result must be:

'[Foo] bar1' -> nothing to do
'[foo] bar2' -> '[Foo] bar2'
'bar3' -> '[Foo] bar3'

CodePudding user response：

You can use startswith to check if the prefix is there (case insensitive; no regex needed). If so, remove it. In all cases, add the prefix (again), but in its proper capitalisation.

def prefixwith(s, prefix):
    if s.upper().startswith(prefix.upper()):
        s = s[len(prefix):].lstrip()  # Remove existing prefix
    return prefix   ' '   s

Call as follows:

prefix = '[Foo]'
print(prefixwith('[Foo] bar1', prefix))
print(prefixwith('[foo] bar2', prefix))
print(prefixwith('bar3', prefix))

CodePudding user response：

Here is one way to do it using re.subn(). It's the same as re.sub, but returns also the number of changes made.

We replace each occurrence of [foo] and [Foo] at the beginning of the string, then we look at the number of matches. If it's 0, then we add [Foo] to the beginning of the string.

import re

data = ['[Foo] bar1', '[foo] bar2', 'bar3']

output = []
for element in data:
    new_string, count = re.subn(r'^\[[Ff]oo]', '[Foo]', element)
    if not count:
        new_string = f"[Foo] {new_string}"
    output.append(new_string)

print(output)

Output:

[
    '[Foo] bar1',
    '[Foo] bar2',
    '[Foo] bar3'
]

---

The regex used is the following:

^\[[Ff]oo]

• ^: asserts position at start of a line.
• \[: matches [.
• [Ff]: matches either f or F.
• oo]: matches oo]

CodePudding user response：

You could replace an optionally existing one:

re.sub(r'^(?i:\[Foo] )?', '[Foo] ', string)

Demo:

import re

strings = '[Foo] bar1', '[foo] bar2', 'bar3'

for string in strings:
    print(re.sub(r'^(?i:\[foo] )?', '[Foo] ', string))

Output:

[Foo] bar1
[Foo] bar2
[Foo] bar3