Rename a math variable in LaTeX with vim regex -

i need change name of math variable across big latex document, of vim regular expressions, struggling learn.

for example, want change variable labelled 't' 's', example

\begin{equation}     f(x) \leq \sqrt{t(t+1)} \end{equation} 

should turn into

\begin{equation}     f(x) \leq \sqrt{s(s+1)} \end{equation} 

for need search character 't' somewhere between \begin{equation} , matching \end{equation} possibly several line breaks away, character 't' should not part of keyword such \sqrt

i have tried

%s/\\begin{equation}\_.\{-}\(\\\a*\)\@!\zst\ze\_.\{-}\\end{equation}/s/g 

but doesn't work should. besides, pattern matches keywords \sqrt , don't understand why. however, am aware sandwiching match between 2 \_.\{-} doing, won't yield desired result, since won't match multiple occurrences of character 't'.

i prefer pure vim regexp solution, if possible.

note:

since use different environments besides equation, such align, eqnarray, multline, etc., better insert or pattern such as

/\\begin{\(equation\|align\|eqnarray\|multline\)} 

and find matching \end{…}. how accomplish that?

let's build pieces. first, @rbernabe suggests, use start-of-word , end-of-word patterns find "t" entire word:

/\<t\> 

next, replace such on current line, use

:s/\<t\>/s/g 

now, position cursor on \begin{equation} line. range .,/\\end{equation}/ specifies lines , including end of equation environment, so

:.,/\\end{equation}/s/\<t\>/s/g 

will replace single-word "t"s "s" in range. in order repeat command equation environments in file, use :g/\\begin{equation}/{command}:

:g/\\begin{equation}/.,/\\end{equation}/s/\<t\>/s/g 

if want play vimgolf, . optional. finally, if want handle multiple environments, afraid there no way tie variable environment name in \begin{...} closing \end{...}. if none of equation-like environments nested, can use

:g/\\begin{\(equation\|align\|eqnarray\|multline\)}/.,/\\end{\(equation\|align\|eqnarray\|multline\)}/s/\<t\>/s/g 

you can use explicit loop, might if there nested environments:

:let envs = ['equation', 'align', 'eqnarray', 'multline'] :for env in envs :  execute 'g/\\begin{' . env . '}/.,/\\end{' . env . '}/s/\<t\>/s/g' :endfor 

of course, regular expressions not substitute real parsing. long actual latex code similar sample, should work, run trouble if ever have like

\end{equation} ordinary text t in it.