The LaTeX command for backslash (\) is \textbackslash.
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Write \textbackslash in normal text, outside math mode. It is part of core LaTeX, so no extra package is required.
\textbackslash is a text command: inside a formula it needs \text{...} from amsmath. With XeLaTeX or LuaLaTeX you can also type \ directly.
Open this example in the live preview: it renders as you type, with no compiler and no signup.
\P\textbullet\dots-----\textasciitilde\textasciicircum\textasteriskcentered\slash``''\textcopyright\textregistered\texttrademarkText symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \textbackslash in normal text; it needs no extra package. For example, \textbackslash.
None: \textbackslash is built into LaTeX, so no extra package is required.