mjhjmtu

My code goes like this:

cat file.ign | sed 's/^([^A-Za-z0-9]+ )/<ignore>1</ignore>/g'

Yet I receive an error saying :

sed: 1: "s/^([^A-Za-z0-9]+ )/<ig ...": 1 not defined in the RE

Parentheses are literal in basic regular expression (BRE) syntax - to make them denote a capture group, they must be escaped, as ( and )

Additionally, as noted in a comment by @BenjaminW, + is also literal in BRE. GNU sed supports + as a quantifier in BRE:

sed 's/^([^A-Za-z0-9]+ )/<ignore>1</ignore>/g'

(but other implementations might not). Alternatively, turn on extended regular expression (ERE) mode using the -E or -r command line switch as appropriate (check your version's documentation):

sed -E 's/^([^A-Za-z0-9]+ )/<ignore>1</ignore>/g'

or use the POSIX-compliant quantifier 1,

sed 's/^([^A-Za-z0-9]1, )/<ignore>1</ignore>/g'

ASIDE the g (global replacement) modifier won't have any effect here, since ^ anchors the expression to the start of the pattern (which can occur only once per line)

Using GNU sed, you can invoke as sed -E or else you can escape the parentheses in order to get the back-reference working, but if you're not using GNU sed, you can't rely on back references; POSIX sed doesn't (yet) support the -E or -r argument for Extended Regular Expressions (ERE) nor does it support GNU's Basic Regular Expression (BRE) backslash notation. (Here's more on POSIX ERE & BRE, though it doesn't speak in much depth about GNU's BRE implementation allowing escaping into ERE functionality.)

Fortunately, (in this case) you don't need ERE. This should work for you:

sed 's/^[^A-Za-z0-9][^A-Za-z0-9]* /<ignore>&</ignore>/' file.ign

BRE doesn't support the + quantifier, so I needed to double the nonword character set so it would be "one or more" by requiring "one" and then "zero or more" of them. The ampersand (&) denotes the entire match. There's no need to have parentheses around everything in this case. (The beginning-of-line anchor ^ is zero-width and doesn't matter here.)

I removed the cat because it's unnecessary. I also removed your /g modifier because the ^ anchor means this can only match once per line, so you'll never get a second match and therefore a global replacement doesn't do anything.