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20. Regular Expressions

> 20.1 Overview

> > JMeter includes the pattern matching software Apache Jakarta ORO
> > There is some documentation for this on the Jakarta web-site, for example a summary of the pattern matching characters

> > There is also documentation on an older incarnation of the product at OROMatcher User's guide , which might prove useful.

> > The pattern matching is very similar to the pattern matching in Perl. A full installation of Perl will include plenty of documentation on regular expressions - look for perlrequick, perlretut, perlre, perlreref.

> > It is worth stressing the difference between "contains" and "matches", as used on the Response Assertion test element:

> > - "contains" means that the regular expression matched at least some part of the target, so 'alphabet' "contains" 'ph.b.' because the regular expression matches the substring 'phabe'.
> > - "matches" means that the regular expression matched the whole target. So 'alphabet' is "matched" by 'al.*t'.

> > In this case, it is equivalent to wrapping the regular expression in ^ and $, viz '^al.*t$'.

> > However, this is not always the case. For example, the regular expression 'alp|.lp.*' is "contained" in 'alphabet', but does not match 'alphabet'.

> > Why? Because when the pattern matcher finds the sequence 'alp' in 'alphabet', it stops trying any other combinations - and 'alp' is not the same as 'alphabet', as it does not include 'habet'.

> > Note: unlike Perl, there is no need to (i.e. do not) enclose the regular expression in //.

> > So how does one use the modifiers ismx etc if there is no trailing /? The solution is to use extended regular expressions , i.e. /abc/i becomes (?i)abc. See also Placement of modifiers below.

> 20.2 Examples

> > ### Extract single string

> > Suppose you want to match the following portion of a web-page:
> > `name="file" value="readme.txt">`
> > and you want to extract `readme.txt` .
> > A suitable regular expression would be:
> > `name="file" value="(.+?)">`

> > The special characters above are:

> > - ( and ) - these enclose the portion of the match string to be returned
> > - . - match any character
> > - + - one or more times
> > - ? - don't be greedy, i.e. stop when first match succeeds

> > Note: without the ?, the .+ would continue past the first `">` until it found the last possible `">` - which is probably not what was intended.

> > Note: although the above expression works, it's more efficient to use the following expression:
> > `name="file" value="([^"]+)">` where
> > [^"] - means match anything except "
> > In this case, the matching engine can stop looking as soon as it sees the first `"` , whereas in the previous case the engine has to check that it has found `">` rather than say `" >` .

> > ### Extract multiple strings

> > Suppose you want to match the following portion of a web-page:
> > `name="file.name" value="readme.txt"` and you want to extract both `file.name` and `readme.txt` .
> > A suitable reqular expression would be:
> > `name="([^"]+)" value="([^"]+)"`
> > This would create 2 groups, which could be used in the JMeter Regular Expression Extractor template as $1$ and $2$.

> > The JMeter Regex Extractor saves the values of the groups in additional variables.

> > For example, assume:

> > - Reference Name: MYREF
> > - Regex: name="(.+?)" value="(.+?)"
> > - Template: $1$$2$

> > Do not enclose the regular expression in / /

> > The following variables would be set:

> > - MYREF: file.namereadme.txt
> > - MYREF_g0: name="file.name" value="readme.txt"
> > - MYREF_g1: file.name
> > - MYREF_g2: readme.txt
> > These variables can be referred to later on in the JMeter test plan, as ${MYREF}, ${MYREF_g1} etc

> 20.3 Line mode

> > The pattern matching behaves in various slightly different ways, depending on the setting of the multi-line and single-line modifiers. Note that the single-line and multi-line operators have nothing to do with each other; they can be specified independently.

> > ### Single-line mode

> > Single-line mode only affects how the '.' meta-character is interpreted.

> > Default behaviour is that '.' matches any character except newline. In single-line mode, '.' also matches newline.

> > ### Multi-line mode

> > Multi-line mode only affects how the meta-characters '^' and '$' are interpreted.

> > Default behaviour is that '^' and '$' only match at the very beginning and end of the string. When Multi-line mode is used, the '^' metacharacter matches at the beginning of every line, and the '$' metacharacter matches at the end of every line.

> 20.4 Meta characters

> > Regular expressions use certain characters as meta characters - these characters have a special meaning to the RE engine. Such characters must be escaped by preceeding them with \ (backslash) in order to treat them as ordinary characters. Here is a list of the meta characters and their meaning (please check the ORO documentation if in doubt).

> > - ( ) - grouping
> > - [ ] - character classes
> > - { } - repetition
> > - * + ? - repetition
> > - . - wild-card character
> > - \ - escape character
> > - | - alternatives
> > - ^ $ - start and end of string or line

> > Please note that ORO does not support the \Q and \E meta-characters. [In other RE engines, these can be used to quote a portion of an RE so that the meta-characters stand for themselves.]

> 20.5 Placement of modifiers

> > Modifiers can be placed anywhere in the regex, and apply from that point onwards. [A bug in ORO means that they cannot be used at the very end of the regex. However they would have no effect there anyway.]

> > The single-line (?s) and multi-line (?m) modifiers are normally placed at the start of the regex.

> > The ignore-case modifier (?i) may be usefully applied to just part of a regex, for example:

> >     Match ExAct case or (?i)ArBiTrARY(?-i) case

20.6 Testing Regular Expressions

> There is a demo applet for Apache JMeter ORO.

> Another approach is to use a simple test plan to test the regular expressions. The Java Request sampler can be used to generate a sample, or the HTTP Sampler can be used to load a file. Add a Debug Sampler and a Tree View Listener and changes to the regular expression can be tested quickly, without needing to access any external servers.

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