{"id":109429,"date":"2020-02-20T15:47:52","date_gmt":"2020-02-20T15:47:52","guid":{"rendered":"https:\/\/blog.finxter.com\/?p=6316"},"modified":"2020-02-20T15:47:52","modified_gmt":"2020-02-20T15:47:52","slug":"python-regex-greedy-vs-non-greedy-quantifiers","status":"publish","type":"post","link":"https:\/\/sickgaming.net\/blog\/2020\/02\/20\/python-regex-greedy-vs-non-greedy-quantifiers\/","title":{"rendered":"Python Regex Greedy vs Non-Greedy Quantifiers"},"content":{"rendered":"

Ready to earn the black belt of your regex superpower<\/a>? This tutorial shows you the subtle but important difference between greedy and non-greedy regex quantifiers. <\/p>\n

But first things first: what are “quantifiers” anyway?<\/strong> Great question – I’m glad you asked! So let’s dive into Python’s three main regex quantifiers.<\/p>\n

Python Regex Quantifiers<\/h2>\n

The word “quantifier<\/a>” originates from latin: it’s meaning is quantus = how much \/ how often<\/strong>.<\/p>\n

This is precisely what a regular expression quantifier means: you tell the regex engine how often you want to match a given pattern. <\/strong><\/p>\n

If you think you don’t define any quantifier, you do it implicitly: no quantifier means to match the regular expression exactly once.<\/p>\n

So what are the regex quantifiers in Python?<\/p>\n

\n\n\n\n\n\n\n\n\n
Quantifier<\/td>\nMeaning<\/td>\n<\/tr>\n
A?<\/code><\/td>\nMatch regular expression A<\/code> zero or one times<\/td>\n<\/tr>\n
A*<\/code><\/td>\nMatch regular expression A<\/code> zero or more times<\/td>\n<\/tr>\n
A+<\/code><\/td>\nMatch regular expression A<\/code> one or more times<\/td>\n<\/tr>\n
A{m}<\/code><\/td>\nMatch regular expression A<\/code> exactly m times<\/td>\n<\/tr>\n
A{m,n}<\/code><\/td>\nMatch regular expression A<\/code> between m and n times (included)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n

Note that in this tutorial, I assume you have at least a remote idea of what regular expressions actually are. If you haven’t, no problem, check out my detailed regex tutorial on this blog<\/a>.<\/p>\n

You see in the table that the quantifiers ?<\/code>, *<\/code>, +<\/code>, {m}<\/code>, and {m,n}<\/code> define how often you repeat the matching of regex A<\/code>. <\/p>\n

Let’s have a look at some examples—one for each quantifier:<\/p>\n

>>> import re\n>>> re.findall('a?', 'aaaa')\n['a', 'a', 'a', 'a', '']\n>>> re.findall('a*', 'aaaa')\n['aaaa', '']\n>>> re.findall('a+', 'aaaa')\n['aaaa']\n>>> re.findall('a{3}', 'aaaa')\n['aaa']\n>>> re.findall('a{1,2}', 'aaaa')\n['aa', 'aa']<\/pre>\n
In each line, you try a different quantifier on the same text 'aaaa'<\/code>. And, interestingly, each line leads to a different output:<\/p>\n