{"version":"https:\/\/jsonfeed.org\/version\/1","title":"mathspp.com feed","home_page_url":"https:\/\/mathspp.com\/blog\/tags\/regex","feed_url":"https:\/\/mathspp.com\/blog\/tags\/regex.json","description":"Stay up-to-date with the articles on mathematics and programming that get published to mathspp.com.","author":{"name":"Rodrigo Gir\u00e3o Serr\u00e3o"},"items":[{"title":"Remove extra spaces","date_published":"2026-03-02T13:39:00+01:00","id":"https:\/\/mathspp.com\/blog\/remove-extra-spaces","url":"https:\/\/mathspp.com\/blog\/remove-extra-spaces","content_html":"

Learn how to remove extra spaces from a string using regex, string splitting, a fixed point, and itertools.groupby<\/code>.<\/p>\n\n

In this article you'll learn about three different ways in which you can remove extra spaces from the middle of a string.\nThat is, you'll learn how to go from a string like<\/p>\n

string = \"This is  a   perfectly    normal     sentence.\"<\/code><\/pre>\n
to a string like<\/p>\n
string = \"This is a perfectly normal sentence.\"<\/code><\/pre>\n
The best solution to remove extra spaces from a string<\/a><\/h2>\n
The best solution for this task, which is both readable and performant, uses the regex module re<\/code>:<\/p>\n
import re\n\ndef remove_extra_spaces(string):\n    return re.sub(\" {2,}\", \" \", string)<\/code><\/pre>\n
The function sub<\/code> can be used to sub<\/strong>stitute a pattern for a replacement you specify.\nThe pattern \" {2,}\"<\/code> finds runs of 2 or more consecutive spaces and replaces them with a single space.<\/p>\n
String splitting<\/a><\/h2>\n
Using the string method split<\/code> can also be a good approach:<\/p>\n
def remove_extra_spaces(string):\n    return \" \".join(string.split(\" \"))<\/code><\/pre>\n
If you're using string splitting, you'll want to provide the space \" \"<\/code> as an argument.\nIf you call split<\/code> with no arguments, you'll be splitting on all<\/em> whitespace, which is not what you want if you have newlines and other whitespace characters you should preserve.<\/p>\n
This solution is great, except it doesn't work:<\/p>\n
print(remove_extra_spaces(string))\n# 'This is  a   perfectly    normal     sentence.'<\/code><\/pre>\n
The problem is that splitting on the space will produce a list with empty strings:<\/p>\n
print(string.split(\" \"))\n# ['This', 'is', '', 'a', '', '', 'perfectly', '', '', '', 'normal', '', '', '', '', 'sentence.']<\/code><\/pre>\n
These empty strings will be joined back together and you'll end up with the same string you started with.\nFor this to work, you'll have to filter the empty strings first:<\/p>\n
def remove_extra_spaces(string):\n    return \" \".join(filter(None, string.split(\" \")))<\/code><\/pre>\n
Using filter(None, ...)<\/code> filters out the Falsy<\/a> strings, so that the final joining operation only joins the strings that matter.<\/p>\n
This solution has a problem, though, in that it will completely remove any leading or trailing whitespace, which may or may not be a problem.<\/p>\n
The two solutions presented so far — using regular expressions and string splitting — are pretty reasonable.\nBut they're also boring.\nYou'll now learn about two other solutions.<\/p>\n
Replacing spaces until you hit a fixed point<\/a><\/h2>\n
You can think about the task of removing extra spaces as the task of replacing extra spaces by the empty string.\nAnd if you think about doing string replacements, you should think about the string method replace<\/code>.<\/p>\n
You can't do something like string.replace(\" \", \"\")<\/code>, otherwise you'd remove all<\/em> spaces, so you have to be a bit more careful:<\/p>\n
def remove_extra_spaces(string):\n    while True:\n        new_string = string.replace(\"  \", \" \")\n        if new_string == string:\n            break\n        string = new_string\n    return string<\/code><\/pre>\n
You can replace two consecutive spaces by a single space, and you repeat this operation until nothing changes in your string.<\/p>\n
The idea of running a function until its output doesn't change is common enough in maths that they call...<\/p>","summary":"Learn how to remove extra spaces from a string using regex, string splitting, a fixed point, and itertools.groupby.","date_modified":"2026-03-02T16:25:02+01:00","tags":["programming","python","algorithms","regex"],"image":"\/user\/pages\/02.blog\/remove-extra-spaces\/thumbnail.webp"},{"title":"TIL #131 \u2013 Change casing in search & replace","date_published":"2025-09-03T00:51:00+02:00","id":"https:\/\/mathspp.com\/blog\/til\/change-casing-in-search-and-replace","url":"https:\/\/mathspp.com\/blog\/til\/change-casing-in-search-and-replace","content_html":"
Today I learned you can change the casing of matched groups when doing a search & replace in VS Code with regex.<\/p>\n\n
VS Code has a search & replace feature that lets you use regex to look for patterns and then reference groups in the replacement...\nBut it lets you do something else that's really cool.<\/p>\n
Changing casing with special sequences<\/a><\/h2>\n
When you are replacing groups, you can use special sequences to change the casing of the group you're inserting, according to the following table:<\/p>\n\n\n\n\n\n\n\n\n
Sequence<\/th>\nEffect<\/th>\n<\/tr>\n<\/thead>\n
\\u<\/code><\/td>\nUppercase the first letter<\/td>\n<\/tr>\n
\\U<\/code><\/td>\nUppercase the whole group<\/td>\n<\/tr>\n
\\l<\/code><\/td>\nLowercase the first letter<\/td>\n<\/tr>\n
\\L<\/code><\/td>\nLowercase the whole group<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
The picture below shows an example of a search & replace operation where I looked for the text \u201call in one go\u201d.\nI enclosed that in a regex group and I'm replacing it with the pattern \\U$1<\/code>, which means the replacement would be the all-uppercase string \u201cALL IN ONE GO\u201d:<\/p>\n
\"Screenshot
Search and replace with \\U.<\/figcaption><\/figure>\n
Pretty nifty, right?<\/p>\n
The same thing in Python<\/a><\/h2>\n
The module re<\/code> in Python supports searching and replacing with the function re.sub<\/code> but it doesn't let you do the same case-changing operations with special sequences.\nInstead, you have to use the fact that re.sub<\/code> supports dynamic string replacements<\/a> and then implement the logic yourself.<\/p>\n
First, you can't use the string methods upper<\/code> and lower<\/code> directly for \\U<\/code> and \\L<\/code>; you have to grab the text from the object Match<\/code>.\nYou also have to pick the string apart to implement the \\u<\/code> and \\l<\/code>:<\/p>\n
def all_upper(match):  # \\U\n    return match.group(0).upper()\n\ndef first_upper(match):  # \\u\n    s = match.group(0)\n    return s[0].upper() + s[1:]\n\ndef all_lower(match):  # \\L\n    return match.group(0).lower()\n\ndef first_lower(match):  # \\l\n    s = match.group(0)\n    return s[0].lower() + s[1:]<\/code><\/pre>\n
Here's an example:<\/p>\n
# E.g., same behaviour as \\U$0 in VS Code:\nre.sub(\n    \"all in one go\",  # pattern to search for\n    all_upper,  # dynamic replacement\n    \"... all in one go ...\",  # source text\n)  # -> '... ALL IN ONE GO ...'<\/code><\/pre>\n\n\n\n\n\n\n\n\n
VS Code sequence<\/th>\nPython function<\/th>\n<\/tr>\n<\/thead>\n
\\u<\/code><\/td>\nfirst_upper<\/code><\/td>\n<\/tr>\n
\\U<\/code><\/td>\nall_upper<\/code><\/td>\n<\/tr>\n
\\l<\/code><\/td>\nfirst_lower<\/code><\/td>\n<\/tr>\n
\\L<\/code><\/td>\nall_lower<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>","summary":"Today I learned you can change the casing of matched groups when doing a search & replace in VS Code with regex.","date_modified":"2025-10-20T22:34:56+02:00","tags":["regex","productivity","vscode","python"],"image":"\/user\/pages\/02.blog\/04.til\/131.change-casing-in-search-and-replace\/thumbnail.webp"},{"title":"TIL #128 \u2013 Matching prefixes and suffixes with regex","date_published":"2025-07-23T13:17:00+02:00","id":"https:\/\/mathspp.com\/blog\/til\/matching-prefixes-and-suffixes-with-regex","url":"https:\/\/mathspp.com\/blog\/til\/matching-prefixes-and-suffixes-with-regex","content_html":"
Today I learned how to use \\b<\/code> and \\B<\/code> to match prefixes and suffixes with regex.<\/p>\n\n
Matching prefixes and suffixes with regular expressions<\/a><\/h2>\n
The special characters \\b<\/code> and \\B<\/code> go hand in hand in regular expressions:<\/p>\n