As Delft is present in the given string, the given condition is satisfied, and … I'm sure this is simple, I just can't get my brain around it. @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. Use the == operator with the [[command for pattern matching. The regex engine does not permanently substitute back-references in the regular expression. 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. Using Regex Operator # Another option to determine whether a specified substring occurs within a string is to use the regex operator =~. -w, --word-regexp Select only those lines containing matches that form whole words. When this operator is used, the right string is considered as a regular expression. Via expr function; a part of pattern matching operators in the form ${param:offset[:length}. I know that BASH =~ regex can be system-specific, based on the libs available -- in this case, this is primarily CentOS 6.x (some OSX Mavericks with Macports, but not needed) Thanks! I mean, i´d like to extract the string file.txt from the string: This is the file.txt this regex matching on the grep command fails all the time, even if the line contains F08R16 pattern. Bash substring with regular expression, In a bash script, I´d like to extract a variable string from a given string. (POSIX allows either behavior.) substr STRING POSITION LENGTH Returns the substring of STRING beginning at POSITION with length at most LENGTH. This is the same as STRING : REGEX. I'd like to be able to match based on whether it has one or more of those strings -- or possibly all. In the first echo statement substring ‘*.’ matches the characters and a dot, and # strips from the front of the string, so it strips the substring “bash.” from the variable called filename. The period followed by an asterisk . The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character. SunOS and other 'expr''s treat these as regular characters. string1 =~ regex- The regex operator returns true if the left operand matches the extended regular expression on the right. It checks if the string has substring Delft in it or not. When this operator is used, the right string is considered as a regular expression. * is the regex expression to be matched, which says match any string, zero or more characters, before and after Delft.. It will use the last match saved into the back-reference each time it … Linux bash provides a lot of commands and features for Regular Expressions or regex. In second echo statement substring ‘. But if I want to get the substring of the filename that matches the wildcard I have to jump through some ugly hoops: for fname in doc-*.txt; do wildcard=${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]. * matches zero or more occurrences any character except a newline character. The period followed by an asterisk . * matches zero or more occurrences any character except a newline character. Similarly, it must be either at the end of the line or followed by a … Bash provides two implementation of substr function which are not identical:. ; 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. 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