正则表达式，用于捕获和替换模式中的数字

Question

I would like to replace the 3rd argument in the string with a new number (let's say 100). The matched string always starts with function , with the first argument either true or false , and with the number as second argument.

                                               Expected
                    |                             |
                    v                             v
'function(true, 0, 15)'   --> 'function(true, 0, 100)'  
'function(false, 0, 23)'  --> 'function(false, 0, 100)'

I have been reading the related posts but I believe I must have misunderstood some regex concept. The following code is that I had tried but it always replaces the whole string:

import re
string = 'function(true, 0, 15)'
regex = re.compile('function\([a-zA-Z]*, [0-9]*, ([0-9]*)\)')
res = re.sub(regex, '100', string)

print(res) # 100
           # Expected: function(true, 0, 100)

Question: Could you point me out why the above code doesn't work? How would I write the code to achieve the expected result?

Answer 1

As the number you are trying to replace is just followed by a closing parenthesis ) , you can just use this \\d+(?=\\s*\\)) regex and replace it by 100 or whatever value you want. Try these Python codes,

import re
string = 'function(true, 0, 15)'
regex = re.compile(r'\d+(?=\s*\))')
res = re.sub(regex, '100', string)

print(res)

Prints,

function(true, 0, 100)

Also, the reason why your code isn't working as expected and is replacing whole of your string with 100 because the way you've written your regex, it matches your whole input and re.sub function replaces what all matches with second argument and hence all your input gets replaced with 100 . But instead what you want is, to just replace the third argument with 100 hence the way you should write your regex, should only match the third argument value, like demonstrated in below regex demo,

Regex Demo matching only what you want to replace

And your current regex matches whole of your input as shown in below demo,

Regex Demo with your regex matching whole input

Also, in case you feel better and you want to match whole input and then selectively replace only third argument, you can use this regex to capture the function name and first two parameters in group1 like you wanted to capture in your original regex,

(function\([a-zA-Z]*, [0-9]*, )[0-9]*\)

and replace it with \\g<1>100) where \\g<1> references the value captured in group1 and further it is replaced with 100)

Regex Demo with full match and selected replacement

Answer 2

This expression also might work:

(?:\d+)(\))

which has a non-capturing group with our desired digits (?:\\d+) , followed by a right boundary (\\)) , which we can replace it with our new number and $1 .

Test

# coding=utf8
# the above tag defines encoding for this document and is for Python 2.x compatibility

import re

regex = r"(?:\d+)(\))"

test_str = "function(true, 0, 15)"

subst = "100\\1"

# You can manually specify the number of replacements by changing the 4th argument
result = re.sub(regex, subst, test_str, 0, re.MULTILINE)

if result:
    print (result)

# Note: for Python 2.7 compatibility, use ur"" to prefix the regex and u"" to prefix the test string and substitution.

Demo

Answer 3

An alternative, you can print everything before the match and everything after the match, then print it out with the new result like so:

regex = re.compile( '(function\([a-zA-Z]*, [0-9]*, )([0-9]*)(\))' )
res = re.sub( regex, r'\1 100\3', string )

Basically, I placed parenthesis around the text before the expected match and after the expected match. Then I print it out as \\1 (first match) 100 (new text) \\3 (third match).

The reason why I propose this particular expression is in case OP specifically needs to only match strings that also contain the preceding "function(" section (or some other pattern). Plus, this is just an extension of OP's solution, so it may be more intuitive to OP.