Linux bash provides a lot of commands and features for Regular Expressions or regex. Via expr function; a part of pattern matching operators in the form ${param:offset[:length}. Using Regex Operator# Another way is to use the regex operator =~ to check whether a specified substring occurs within a string. This is the same as STRING : REGEX. The period followed by an asterisk . The regex engine does not permanently substitute back-references in the regular expression. * matches zero or more occurrences any character except a newline character. * matches zero or more occurrences any character except a newline character. In the first echo statement substring ‘*.’ matches the characters and a dot, and # strips from the front of the string, so it strips the substring “bash.” from the variable called filename. Bash version 3, present on must current Linux distributions, addresses this lack by allowing regular expression matching. I'd like to be able to match based on whether it has one or more of those strings -- or possibly all. In second echo statement substring ‘. The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character. Use the == operator with the [[command for pattern matching. I'm sure this is simple, I just can't get my brain around it. SunOS and other 'expr''s treat these as regular characters. (POSIX allows either behavior.) ; I recommend using the second one, as this is more compact notation and does not involves using external function expr.It also looks more modern, as if inspired by Python, although its origin has nothing to do with Python. string1 =~ regex- The regex operator returns true if the left operand matches the extended regular expression on the right. Bash substring with regular expression, In a bash script, I´d like to extract a variable string from a given string. It will use the last match saved into the back-reference each time it … The annoying issue is that GNU grep (or glibc) supports some PCRE-like atoms, at least \w and \s when interpreting ERE, and in that context they very much are nonstandard. It checks if the string has substring Delft in it or not. match STRING REGEX An alternative way to do pattern matching. I know that BASH =~ regex can be system-specific, based on the libs available -- in this case, this is primarily CentOS 6.x (some OSX Mavericks with Macports, but not needed) Thanks! @DanielFarrell, the standard in this case is what POSIX specifies, and it doesn't know about \d.Though you're right in that PCRE are rather standard, or in the least well-defined. The period followed by an asterisk . But if I want to get the substring of the filename that matches the wildcard I have to jump through some ugly hoops: for fname in doc-*.txt; do wildcard=${fname#doc-} wildcard=${wildcard%.txt} echo input: ${fname} output: output-${wildcard}.results done ... is the first group in the regex, it's in BASH_REMATCH[1]. As Delft is present in the given string, the given condition is satisfied, and … When this operator is used, the right string is considered as a regular expression. * is the regex expression to be matched, which says match any string, zero or more characters, before and after Delft.. Here, .*Delft. grep , expr , sed and awk are some of them.Bash also have =~ operator which is named as RE-match operator.In this tutorial we will look =~ operator and use cases.More information about regex command cna be found in the following tutorials. substr STRING POSITION LENGTH Returns the substring of STRING beginning at POSITION with length at most LENGTH. 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