I assume you used something like sed 's/al/LONGsomething/g' file
The "problem" is /g means global replacement, thus sed will attempt to replace all occurences of the al string, no matter where in the line.

If you remove g from the sed command like this sed 's/al/LONG/' file sed will only replace the first occurence of al.
By using a caret ^, you explicitely tell sed to match al at the beginning of the line:

Code:

sed 's/^al/LONG/' file

The above would *almost* do what you want... Why almost? It will replace all al's which are at the beginning of the line, so al gets LONG, and al2h6 gets LONG2h6 You get the idea.

THIS is what you want for al (it acts like replace al only found as single string, not as part of some other string too)

Code:

sed 's/^al /LONG0003/' file

and this for al2h6

Code:

sed 's/^al2h6 /LONG0003/' file