Here, we outline the special characters for grep. Note that in egrep (which uses extended regular expressions), which actually are no more functional than standard regular expressions if you use GNU grep ) , the list of special characters increases ( | in grep is the same as \| egrep and vice versa, there are also other differences. Check the man page for details ) The following characters are considered special and need to be "escaped":
Note that a $ sign loses its meaning if characters follow it (I think) and the carat ^ loses its meaning if other characters precede it.? \ . [ ] ^ $
Square brackets behave a little differently. The rules for square brackets go as follows:
- A closing square bracket loses its special meaning if placed first in a list. for example
[]12]matches ] , 1, or 2. - A dash - loses it's usual meaning inside lists if it is placed last.
- A carat ^ loses it's special meaning if it is not placed first
- Most special characters lose their meaning inside square brackets
- * if at the beginning of the regular exps, lose its meaning.
A regular expression may be followed by one of several repetition operators:
? The preceding item is optional and matched at most once.
* The preceding item will be matched zero or more times.
+ The preceding item will be matched one or more times.
{n} The preceding item is matched exactly n times.
{n,} The preceding item is matched n or more times.
{n,m} The preceding item is matched at least n times, but not more than m times.
In basic regular expressions the metacharacters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions
\?, \+, \{, \|, \(, and \).
only . and * have special meanings in grep
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ReplyDeleteThe metacharacters that do not have special meaning in grep:
ReplyDelete1. the repetition operators: ?, +, {}.
* has special meaning.
2. The anchoring operators: ^ and $ have special meaning(lose meaning if not for positioning)
3. Alternation | have no special meaning
4. bracket expression [] has special meaning
5. Paretheses (): no special meaning
6. Operator . and \ have special meanings
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Operators ., *, ^, $, [], has special meaning, also, \ has
In item 1, the repetition operators: ?, +, {} have no special meaning. * has special meaning.
ReplyDeleteSpecial groups such as [[:num:]] and \w, word boundaries -w in grep
The main differences between the vi/sed and the grep/gawk:
ReplyDeleteIn vim/sed, the \(, \) is used for storing pattern for later replay, grep/gawk does not have this.
In grep/gawk, the \(, \) is used for alternations.
vim and sed difference: vim has \< and \>, indicating word boundaries.
gawk and grep differences: in grep {} are common characters, to use the repetition meaning, we need the \{ and \}. In gawk, the {} are special characters
vim and sed difference: vim has \< and \>, indicating word boundaries. sed also has the same thing. This is not a difference
ReplyDelete