Python中匹配第一个点之前字符串的正则表达式方案

Hey there! Let's sort out this regex issue for you.

First, let's break down why your current regex r'(.*)\.(.*)' isn't doing what you want:

• The greedy .* matches as much as possible, so it actually captures everything up to the last dot (not the first). For example, with yahoo.co.uk, it would grab yahoo.co instead of just yahoo.
• It doesn't distinguish between domain names and IP addresses, so it will happily capture the first octet of an IP like 192 from 192.168.1.1.

Improved Regex #

Here's a regex that captures the part before the first dot only if the input isn't a full IP address:

r'^(?!(?:\d{1,3}\.){3}\d{1,3}$)([^.]+)\.'

Let's break down each component:

• ^ : Anchors the match to the start of the string.
• (?!(?:\d{1,3}\.){3}\d{1,3}$) : Negative lookahead that skips any string formatted as a valid IP address (four dot-separated numeric segments).
• ([^.]+) : Captures one or more characters that are not dots—this stops at the first dot, exactly what you want for prefixes like google or yahoo.
• \. : Matches the first dot (we don't need to capture this, just use it to mark the boundary).

How It Works #

• For valid domains like google.com or yahoo.co.uk, it captures google and yahoo respectively.
• For full IP addresses like 192.168.1.1, the negative lookahead prevents the regex from matching at all, so you won't get unwanted captures like 192.
• It also works for domains with numeric prefixes (like 123.com), which are valid and should be captured.

Quick Test Cases #

Alternative: If You Want to Avoid Numeric-Only Prefixes #

If you don't need to support numeric-only domains (like 123.com) and just want to exclude IP-like segments, you can use this simpler regex to ensure the prefix has at least one letter:

r'^([a-zA-Z0-9-]*[a-zA-Z][a-zA-Z0-9-]*)\.'

Hope this solves your problem!

内容的提问来源于stack exchange，提问作者Jasmin