How to add a letter in front of file names in a dir when file names have digit(s) at the beginning

With zsh instead of bash, to rename <digits>.svg to A<digits>Solid.svelte:

autoload -Uz zmv 

To autoload the zmv batch-renaming function (best in ~/.zshrc)

zmv -v '(<->).svg' 'A${1}Solid.svelte' 

With perl-based renames, you can do the same (though without the safeguards of zmv) with:

rename -v 's/^(\d+)\.svg\Z/A${1}Solid.svelte/' [0-9]*.svg 

The [0-9]*.svg expands to file names that start with a character in the 0 to 9 range (which includes but in some shells including bash is not limited to 0123456789) and end in .svg, but rename will only rename those that are made only of ASCII decimal digits (0123456789) followed by .svg.

Using a [0-9]*.svg glob also means the file names won't start with -, which means we don't need the -- which not all variants of perl-based rename support (while for others, omitting it introduces a command injection vulnerability!). Not all support -v either.

If the intent is to capitalise the root names of the files and append Solid.svelte, but also prepend A to file names starting with a digits, for instance for foo-bar.svg to become FooBarSolid.svelte, 0.svg A0Solid.svelte and 0foo-bar.svg A0FooBarSolid.svelte as your code seems to be trying to do, you'd do:

zmv -v '([0-9]#)(*).svg' '${1:+A$1}${${(C)2}//-}Solid.svelte' 

Or with rename:

rename -v 's{^\./(\d*)(.*)\.svg\Z}{ ($1 eq "" ? "" : "A$1") . ($2 =~ s/\w+/\u$&/gr =~ s/-//gr) . "Solid.svelte" }ge' ./*.svg 

Bearing in mind that contrary to zmv, it only works properly on ASCII text (no captitalising of éric to Éric for instance).

Or you can extend the code you already have if you're happy with it by adding ; $_ = "A$_" if /^\d/ to prepend a A if the file name starts with a decimal digit.