Regex overlapping

Question

I want to capture all occurrences of a letter, followed by a digit, but no letter before the above letter. So, there is a f9 match in f9, 3f9, f99, but not in af9. I have the following regex: ([^a-z]|^)([a-z])(\d).

Why doesn't it match both f9 and x0 in f9x0 and how to achieve this? It matches only f9. In f9 x0 there are two matches.

Thank you.

Enter your regular expression into a website such as regex101.com and see what your regex means. — AdrianHHH, May 15 '21 at 12:07
There is no character before the`x` but you require one. You are looking for a neg. lookbehind. — Jan, May 15 '21 at 12:08
Does this answer your question? [Learning Regular Expressions](https://stackoverflow.com/questions/4736/learning-regular-expressions) — AdrianHHH, May 15 '21 at 12:09
`all occurrences of a symbol` ... `f` is not a symbol, it's a word character. — Tim Biegeleisen, May 15 '21 at 12:12

score 1 · Answer 1 · answered May 15 '21 at 12:12

1

There is no character before the x but you require one. You are looking for a lookbehind:

(?:^|(?<![a-z]))[a-z]\d

See a demo on regex101.com.

answered May 15 '21 at 12:12

Jan

42,290
8
54
79

There is `9` before `x` and it is not a letter, as required. – Paul R May 15 '21 at 13:13
1

Reading the question, I think this should be ok right? If I am not mistaken, you can shorten it to `(?<![a-z])[a-z]\d` – The fourth bird May 15 '21 at 17:03

score 1 · Accepted Answer · answered May 15 '21 at 12:37

1

As you asked it:
"capture all occurrences of a symbol, followed by a digit, but no symbol before the above symbol"

/(?<=\d|^)\w\d/g

Match:

f9      >> f9
f9x0    >> f9, x0
af9    
3f9     >> f9

Regex101.com demo

answered May 15 '21 at 12:37

Roko C. Buljan

196,159
39
305
313

@AndrewFount first of your regex has too many matching groups making the result quite odd: https://regex101.com/r/h07ed8/1 - Secondly the option `^`(*start of string*) in ([^a-z]|^) already **matched and consumed** the first match `f9` - and will not proceed further to `x0`. – Roko C. Buljan May 15 '21 at 13:33
First capture is `f9` (with the start of the string). The second match should be `9x0`. It doesn't work because `9` is already captured, correct? – Paul R May 15 '21 at 13:48
@AndrewFount Yes. If by contrast you use a non-consuming Positive Lookbehind `(?<=` the thing will work as intended for the next group `x0` as well. – Roko C. Buljan May 15 '21 at 14:03

Regex overlapping

2 Answers2