xstring – String manipulation for (La)TeX

The pack­age pro­vides macros for ma­nip­u­lat­ing strings — test­ing a string’s con­tents, ex­tract­ing sub­strings, sub­sti­tu­tion of sub­strings and pro­vid­ing num­bers such as string length, po­si­tion of, or num­ber of re­cur­rences of, a sub­string.
The pack­age works equally in Plain TeX and LaTeX (though e-TeX is al­ways re­quired). The strings to be pro­cessed may con­tain (ex­pand­able) macros.