> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Perform Basic File Management Part 2 Regular expressions 2
> This lesson covers extended regular expressions in Linux, focusing on text search techniques using tools like grep and egrep.
In this lesson, we’ll master **extended regular expressions** (`ERE`) using Linux tools like `grep -E` and `egrep`. You’ll learn how to apply quantifiers, character classes, alternation, and grouping to perform powerful text searches.
## Enabling Extended Regex: `-E` vs `egrep`
By default, `grep` treats metacharacters such as `+`, `?`, `{}`, `|`, and `()` literally. To activate these operators without escaping them, add the `-E` option or use `egrep`:
```bash theme={null}
$ grep -Er '0+' /etc/
/etc/pnm2ppa.conf:#colorshear 0
/etc/pnm2ppa.conf:#blackshear 0
...
/etc/subuid:aaron:100000:65536
```
Or simply:
```bash theme={null}
$ egrep -r '0+' /etc/
```
On some systems, `egrep` is a deprecated symlink to `grep -E`. The two commands are functionally equivalent.
## Quantifiers at a Glance
| Quantifier | Description |
| ---------- | ------------------------------- |
| `x+` | One or more of `x` |
| `x?` | Zero or one of `x` |
| `x{n}` | Exactly *n* occurrences of `x` |
| `x{n,}` | At least *n* occurrences of `x` |
| `x{,m}` | Up to *m* occurrences of `x` |
| `x{n,m}` | Between *n* and *m* occurrences |
### `{n,}` – At Least *n* Matches
Find lines with **three or more** consecutive zeros:
```bash theme={null}
$ egrep -r '0{3,}' /etc/
/etc/vmware-tools/.../xmlidsig-core-schema.xsd:
grep: /etc/firewalld: Permission denied
...
```
### `{,m}` – At Most *m* Matches
Match up to **three zeros** (including none):
```bash theme={null}
$ egrep -r '10{,3}' /etc/
/etc/pmn2ppa.conf:# valid blackness choices are 1 2 3 4
...
```
### `{n}` – Exactly *n* Matches
Search for exactly **three zeros**:
```bash theme={null}
$ egrep -r '0{3}' /etc/
/etc/vmware-tools/.../xmlidsig-core-schema.xsd:
...
```
### `?` – Optional Element
Make the preceding character optional (0 or 1):
```bash theme={null}
$ egrep -r 'disable' /etc/ # matches "disable"
/etc/some.conf:# disable feature X
$ egrep -r 'disabled?' /etc/ # matches "disable" or "disabled"
/etc/other.conf:# disabled by default
```
### `{n,m}` – Range of Matches
Match between 3 and 5 zeros:
```bash theme={null}
$ egrep -r '0{3,5}' /etc/
/etc/example.conf:# found in 0000 sequence
...
```
## Alternation with `|`
Use `|` to choose between patterns. To match `enabled` or `disabled`:
```bash theme={null}
$ egrep -r 'enabled|disabled' /etc/
/etc/tuned/tuned-main.conf:# If enabled, re-apply sysctls.
/etc/vmware-tools/tools.conf.example:# disabled.
```
Combine `?` to make the final letter optional:
```bash theme={null}
$ egrep -ir 'enabled?|disabled?' /etc/
/etc/nanorc:## Enable vim-style lock-files.
/etc/nanorc:## To make sure an option is disabled, use "unset