re = /^([A-Za-z0-9$\/#]+?)[\/_].*/m
str = '#Tableau_6_sid:15251a_4i_2a
/15722c29d_PhotoImageCameraPro
JAVA/15722c29d_PhotoImageqeeee
ABC_CUST_INFO_D
BBC_CUST_ENTRY_F
FHS_PRDCT_STST_T
GHS_INVTR_CD_DRY'
subst = '\\1'
result = str.gsub(re, subst)
# Print the result of the substitution
puts result
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Ruby, please visit: http://ruby-doc.org/core-2.2.0/Regexp.html