Extract numbers after trigger sign from string variable

[Rike Lich]

Dear Statalist,
I have the following data:

* Example generated by -dataex-. To install: ssc install dataex
clear
input str397 SHORTTITLE
"Eine kurze discourse of the  the person of Prince Pi of Orange.Londorf, Thomas Dart para T. C. [=Thomas Chard] W. B. [=William Broome], 1882"              
"Eine Erklärung of the queenes majestät will gegen den Überschuss of appLondorf, deputies Christopher Barker, 1887 (=1888)"                              
"Ein wahrer Report des Erfolgs of Antwarpe zum Prinz von Parma, which was on the seaventeenth of august last past.Amsterdam (=England), Jacob Watleer, 1885"
"von den drei ce qui s'est passé entre l'armee TurksLyon, Guichard Jullie et Tim Ammer, 1897 (=1898)."                                                      
"Eine Veröffentlichung für den Kunden oder Subsudität all such sorts of silkss, wie von der Majestät bestätigt s ...London, Robert Barker, 1800 [=1801]"
"Allergiesn und sterben göttlich legisInstrument ex sanctis et laudatis Autorensammlung Para , in ooffiziel Jos Bade, 1819 (=1820 n.s.)."                  
end

I would like to extract the year that is indicated after the “=” and generate a new variable called yearfix; I would like a missing value in case there is no “=XXXX”.

For example:
Yearfix
.
1888
.
1898
1801
1820


Thank you very much!!!!!

Best,
Rike

[Andrew Musau]

gen wanted = substr(SHORTTITLE , strpos(SHORTTITLE , "=1"), .)
replace wanted= substr(wanted,2,4)
destring wanted, replace

[Robert Picard]

You can also use a regular expression to match a "=" followed by 4 digits.

gen year = real(ustrregexs(1)) if ustrregexm(SHORTTITLE,"=([0-9]{4})")

[Rike Lich]

Dear Robert, Dear Andrew,

thank you for your solutions. Both commands worked pretty well .
I really appreciate your help!
Best,
Rike