#grep #tips
The -o option in grep stands for "only matching." When this option is used, grep will print only the parts of the lines that match the specified pattern, rather than the entire line. This is particularly useful when you want to extract specific substrings from lines of text.

The -E option in grep enables the use of Extended Regular Expressions (ERE) in the pattern. This allows for more powerful and flexible pattern matching compared to basic regular expressions (BRE), which is the default behavior of grep.

### Key Differences with Extended Regular Expressions

When using -E, you can utilize additional metacharacters and constructs that are not available in basic regular expressions. Here are some of the key features of ERE:

1. Metacharacters: In ERE, the following metacharacters are treated as special without needing to be escaped:
- + : Matches one or more occurrences of the preceding element.
- ? : Matches zero or one occurrence of the preceding element.
- | : Acts as a logical OR between expressions.
- () : Groups patterns for applying quantifiers or for alternation.

2. Example Usage:
- To match either "cat" or "dog", you can use:

     grep -E "cat|dog" filename
     

- To match one or more digits, you can use:

     grep -E "[0-9]+"
     

3. Combining with Other Options: You can combine -E with other options like -i for case insensitivity or -o to print only the matching parts:

   grep -Eio "cat|dog" filename