Regex usage in grep

Question

Im trying to search for a pattern in a file as follows:

SRC_ERROR_CODE=105
SRC_ERROR_CODE=106
...

To achieve this following is the grep statement used:

grep -io "[a-z]*_error_code=[0-9]*" events.log

However i was wondering if instead of using the "*" which fetches 0 to n occurrences of the preceding matched character, the "+" should fetch the same results as well as below:

grep -io "[a-z]+_error_code=[0-9]+" events.log

But, this doesn't seem to work. Could you please guide as to why it doesn't.

Thanks

Im not sure what your question is. + will select 1 to N occurrences, and unless specified is greedy in terms of finding matches. I think you need to give more information. Why doesnt it work? Is it returning the wrong set, or is it returning an error, or what? — Fallenreaper
– Fallenreaper, Commented Dec 21, 2015 at 2:18

that other guy · Accepted Answer · 2015-12-21 02:25:43Z

2

In POSIX Basic Regular Expressions (BRE), the default regex dialect used by grep, + just matches itself.

In POSIX Extended Regular Expressions (ERE) and Perl Compatible RegEx (PCRE), + matches 1 or more of the preceding atom.

You can ask grep to use ERE with the -E option:

$ echo "foo baaar" | grep -o -E 'a+'
aaa

answered Dec 21, 2015 at 2:25

that other guy

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mklement0 · Accepted Answer · 2015-12-21 04:08:54Z

0

The + in regex matches one or more character in Extended Regular Expression (ERE - this is egrep)

In Basic regular expression (BRE) you have to escape the + sign.

Since you are using grep, you have to escape the +. Therefore, use \+ instead of +.

If you are using egrep, you can use the unescaped +

edited Dec 21, 2015 at 4:08

mklement0

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answered Dec 21, 2015 at 2:30

repzero

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1 Comment

mklement0 Over a year ago

These days, egrep is just an (effective) alias for grep -E (similarly, fgrep maps onto grep -F). For long-term POSIX compliance it's better to use grep with options. While both BSD grep and GNU grep do support \+ in BREs, it's important to note that \+ in BREs is not POSIX-compliant and other tools, such as sed on BSD-like platforms, do not support it; the POSIX-compliant equivalent of the + ERE construct is the - somewhat cumbersome - \{1,\}.

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