With GNU grep, \b matches the empty string at the edge of a word. In the example the \b before the / is not at the edge of a word, because in the example SYSYSID;MANDT;/SIE/AD_Z0M_INDX;/SIE/AD_Z0M_KEY1 it is between a ; and a / . So try:
What operating system and shell are you using? (As Scrutinizer said, the code he suggested with work with GNU grep . If you're not using a Linux system, there is a good chance that the grep you're using doesn't understand \b or \B .)