Python regex matches string it shouldn't

Question

I'm totally lost at how this regex matches this string in python. Could someone make sense of it please?

import re
regex = "^PHP/5.\\{3|2\\}.\\{1|2|3|4|5|6|7|8|9|0\\}\\{1|2|3|4|5|6|7|8|9|0\\}$"
ua = 'Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)'
re.compile(regex).search(ua)

The regex starts with PHP , while the string does not. Shouldn't that simply disqualify a match from happening?

Answer 1

You need grouping (preferably non-capturing) for your alternation:

PHP/5.\\{(?:3|2)\\}.\\{(?:1|2|3|4|5|6|7|8|9|0)\\}\\{(?:1|2|3|4|5|6|7|8|9|0)\\}$
         ^^    ^       ^^                    ^      ^^                    ^

Other wise you will be alternating the entire expression:

• PHP/5.\\\\{3 or
• 2 or
• \\\\}.\\\\{1 or
• 2 or
• 3 or
• 4 or
• 5 match found!

---

^{Think PEMDAS and nested conditionals ( if(a && (b || c)) { } ).}

Answer 2

Your RegEx fails, because | plays an important role here. So, your string is matched for items, like this

• ^PHP/5.\\\\{3

• 2\\\\}.\\\\{

and so on. Since the or matches 5 in 4|5|6 , it actually matches the 5 in Mozilla/5.0 .

You can see online demo and explanation for the same, here .

Debuggex Demo