Which regex can I use to find exactly two sub-strings in a longer string?

Question

I am completely new to RegEx and I am struggling even with a simple case.

I would like to identify the following cases, for example:

IR!GBP!INDEX.GBP
IR!GBP!INDEX.USD

where the sub-string GBP (or USD) appears exactly twice in the bigger string. The second time, it can appear only after the INDEX. sub-string. How can I detect it via RegEx?

The big string is formed as follows: the first bit is always "IR", the second is a currency, the third another string, all of them separated by the exclamation mark.

For example, IR!GBP!COUNTERPARTY.USD or IR!USD!INDEX.GBP should not return a match.

I hope my question is clear, and thanks a lot in advance for your help!

I tried various combinations with [a-zA-Z] and +? but ended up nowhere. I admit my inability!

A few questions: are GPB and USD the only monetary extensions that may be there? Is that extension always followed by a newline? When you mention "bigger string", do you mean the string including the newlines? Is there a specific language you like to use for the regular expression? — 9769953
– 9769953, Commented Nov 5, 2024 at 13:52
Hi! Thanks a lot for your comment. No, GBP or USD is just an extension. My data list contains quite a few currencies, all of them represented by three letters. The "big string" is just the whole string, while GBP and USD are sub-string. My programme will run in C++. — andreasagninese
– andreasagninese, Commented Nov 5, 2024 at 17:49

JvdV · Accepted Answer · 2024-11-05 14:34:46Z

2

To me it seems like the following ticks your boxes:

^IR!([A-Z]+)![A-Z]+\.\1$

See an online demo. The pattern means:

^IR! - Match start of string followed by exactly uppercase 'IR' and an exclamation mark;
([A-Z]+) - A 1st capture group to hold the content of 1+ uppercase characters (your currency);
![A-Z]+\. - A literal '!' before 1+ uppercase chars and a literal (escaped) dot;
\1 - Match the content of what was captured in the 1st group;
$ - End-line anchor.

answered Nov 5, 2024 at 14:34

JvdV

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